A 1993 Explorit Discovery Lesson * Pre/Post Information & Activity Packet

© copyright 1992 Explorit Science Center.
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Boundaries - Keeping the Insides In!

Exhibition date: May 8 - June 6, 1993
Key Ideas:
Skins, Peels, Borders, Boundaries, Shells, Bubbles, Membranes, Exoskeletons

What keeps the insides in, and why?

Skins, peels, borders, boundaries, shells, membranes, exoskeletons, wrappers, scales, films, paint, glazes, cases... and the list goes on! Everywhere that we look, we see things being contained. Our bodies are covered with our largest organ, the skin. Every room in our house provides other examples: peels on bananas in the kitchen; hot water tank in the garage; glaze on a ceramic pot in the living room; pillowcases on our pillows in the bedroom, etc. Even the house itself keeps us in and keeps weather, other people, noise, etc. out.

The question is: what is the purpose of these various borders? Why are they there? What do they do or what do they not do? In this exhibit, we will explore the world of skins, peels, etc. We will uncover some of the reasons why they exist and perform the way they do. Come prepared to delve into things that are Keeping the Insides In.

What will we discover at EXPLORIT!?

You and your students will:
* get better acquainted with skins both familiar and unfamiliar.
* experiment with making some borders, wrappers, etc.
* explore the purposes and/or functions for various skins, peels, etc.
* encounter a vast array of things which keep insides in.

Questions to Tickle Your Mind:

* What does a shaved zebra look like?
* What makes an orange orange?
* Why are there so many different colors of human skin and hair?
* Why can't creatures with exoskeletons grow to the size of humans?
* Why does the surface of a golf ball have all those little dimples on it?

Background Information:

To help you apply the California Science Framework (1990), we have written the key words, or "big ideas," in boldface.

Skins, peels, membranes, borders, etc. come in such a variety of shapes, sizes, purposes, and functions that they could not all be covered in such a small space on a page nor in an entire exhibit. Therefore, we will consider just a few of the myriad examples.

What does our skin do?

Our skin serves many purposes. First it is the body's largest organ covering 12-20 square feet (on an adult) and comprising 12% of body weight (scale). Working with our immune system, the skin is also the body's front line defense against attacks, bacterial or otherwise, from the outside of the body. It is tough enough to resist being easily punctured or damaged, yet it is sensitive enough to feel even the slightest touch.
There are three layers of the skin: the epidermis, or overskin; the dermis, or skin; and the subcutis, or underskin (structure). The epidermis is largely composed of the tough material called keratin, a hard, horn-like protein that also makes up our hair and fingernails. The epidermis is never thicker than 1/25 inch, the thickest skin being on the bottom of the feet and palms of the hands and the thinnest skin on the eyelids.

Both the dermis and the subcutis are highly vascularized in order to maintain the skin itself, but it also provides temperature control for the interior of the body thus helping us to save energy. If the temperature inside the body changes by as little as 1.5deg. F, the body's metabolism changes by as much as 30% (systems and interactions, stability). The dermis and the subcutis also contain a large percentage of fat to help insulate the body, and the dermis holds the sweat glands which cool us down when we get too hot. On hot days when we exert ourselves physically, we can lose as much as 3 gallons of water through sweating. Further, all three layers also protect against dehydration. Our body is 75% water at birth, but the environment in which we live is much drier.

How do other animals keep their insides in?

Other mammals have skin similar to human skin. However, most mammals have more hair than humans do. We have lost much of our hair simply because it is not necessary. Other mammals use their hair, basically just an extension of the epidermis, to provide insulation and to conserve energy.

Reptiles have scales to cover their bodies. In contrast to mammalian skin, reptile scales do not provide a great deal of insulation. Reptiles are ecothermic, or cold-blooded, creatures which means their body temperature is similar to the ambient temperature. Reptile scales are very tough and difficult to puncture, even by sharp teeth, thus protecting the reptile from attack. Also scales do not grow with the reptile. The reptile must shed its old skin and replace it with new skin as the reptile grows.

Birds have specialized scales called feathers. The feathers provide insulation and conserve energy. They also function in other important ways, such as assisting with flight, providing camouflage, and often playing a key role in courting and mating rituals.

Insects have very specialized coverings called exoskeletons. The insect has no internal skeleton like humans do. Instead their hard "shell" provides support for the insect's body, contains the internal workings of the insect, and protects the insect, to some degree, from external attack.

How do plants keep their insides in?

As with animals, plants must also cover themselves. Most plants have a waxy coating on their leaves called the cuticle. The cuticle helps to prevent loss of water, dehydration, and therefore conserves energy for the plant. The thickness of the cuticle varies according to where the plant lives. For example, cacti in the desert areas of the world often have very thick cuticles as one of many adaptive characteristics to prevent loss of water in their arid climates. A large apple tree loses approximately 320 quarts (304 liters) of water in a day through respiration and evaporation, whereas a large saguaro cactus loses less than one glass of water in the same amount of time.

What do size and "skins" have to do with each other?

An interesting side note of the skin discussion considers the size of skin. As animals, plants, and other creatures grow larger, the body surface (length2), or area, increases much more slowly than the body volume (length3). This limits some creatures' size. For example, if an insect were to grow to an extraordinary size, its volume would increase such that its exoskeleton would not be able to support the additonal weight caused by the larger volume. The insect could scarcely move! Further, as the surface area of the insect increases, more water and heat is lost.

This same principle helps to explain why bubbles are spherical. A sphere is the most "economic" shape to contain any volume, hence the bubble forms a sphere instead of a cube to contain its volume of air.


1. Skin Brainstorm

Materials None


Lead a discussion on skins, peels, etc. What is a skin? A peel? A membrane? Encourage your class to think of as many examples of skins, peels, membranes, borders, etc. as they can. Encourage the children to explain why they consider their contribution to be a border or skin. If desired, expand this activity to include consideration of what the purposes or functions of each skin might be.


How many skins, peels, etc. can be thought of?
* How many types of skins can be thought of? How could they be classified or grouped?
* Does every skin have only one purpose?

2. Bark Rubbings


* Paper
* Crayon
* Trees (several different types, if possible)


Take the class outside with several pieces of paper and one crayon per person. Hunt for all different types of trees in the area. At each tree, place a piece of paper on the bark of the tree and rub the crayon gently on the paper. The crayon will only touch part of the bark leaving a pattern resembling the bark on the paper (a rubbing). Record other information about the tree on the same piece of paper such as location, size, color of bark and leaves, and name of tree, if known. Upon returning from the trip, share and discuss the rubbings with each other.


* What functions does the bark perform for the tree?
* What kinds of similarities and differences can be detected in the bark rubbings?
* What could these similarities/differences mean?

3. Potato Cells

(Adapted from: Burnie, D. (1991) How Nature Works. NY: Reader's Digest Association, Inc.)


* 2 potatoes
* Sugar
* 3 dishes
* Spoon
* Knife
* Water


Cut the 2 potatoes in equal halves lengthwise. Discard 1 half of 1 potato (only 3 halves are needed for the activity). Remove a 1 cm strip of peel around the base of each remaining half, and cut a small hollow in the curved side. Put 2 potato halves flat side down in dishes of water. Boil the remaining half for 10 minutes to kill the cells, and then put it in a dish of water. Potato 1 is the control potato. Do nothing to this potato, and compare it to the others at the experiment's conclusion. Potato 2 is the cooked potato (cells are dead). Place it in a dish with water, and put a spoonful of sugar in the hollow. Examine after one day. Potato 3 is the uncooked potato (cells are still living). Put a spoonful of sugar in the hollow and examine after one day.

Note: Osmosis is the process whereby water moves from the outside of a cell to the inside traveling through the cell's specialized membranes. The membranes let water through but not salt or sugar. The water crosses the membrane if there is more salt or sugar on the other side thus equalizing the concentration on either side.


* What happened to each of the potatoes?
* Why are there differences?
* Try the same experiment substituting other things for sugar such as salt, flour, sugar substitute, etc. What happens?

4. Tooth Decay

(Adapted from: Burnie, D. (1991) How Nature Works. NY: Reader's Digest Association, Inc.)


* A discarded tooth (a child's or animal's)
* Cola
* Drinking glass


Enamel is the extremely hard substance that covers our teeth. Normally our saliva helps to prevent tooth decay which can develop when bacteria in our mouths eat sugar and turn it into acid. However, cola drinks attack our teeth directly. Put the tooth in the cola drink and leave it for at least 24 hours. Take it out and examine after 24 hours.


* Are there any differences?
* What happens if the tooth is left in for more than 24 hours?
* How does this change the class' opinions about drinking cola drinks?

5. Slimy Sponge Border

(Adapted from: Ranger Rick's Nature Scope: Discovering Deserts. (1986).


* Water
* Dry sponge
* Petroleum jelly
* 2 margarine tubs


Pour 1/4 cup (60 ml) of water into each margarine cup. Cut the sponge in half. (Both pieces should be exactly the same size.) Cover one side and all 4 edges with petroleum jelly. Leave one side uncovered. Then lay it in the dish of water with the ungreased side down. Put the other sponge piece in the other tub. Watch as the sponges soak up water. Then keep track of how long it takes for the sponges to dry out. (This may take several days.)


* Which sponge dried out faster? Why?
* What connections can be drawn between this activity and plants?
(Hint: Plants have a waxy cuticle on their leaves that helps to prevent loss of water through evaporation.)

6. Seeing Inside a Shell

(Adapted from: Burnie, D. (1991) How Nature Works. NY: Reader's Digest Association, Inc.)


* Spiral shell
* Coarse sandpaper


Grip the shell tightly. Rub the side of the shell vigorously on the sandpaper. As the shell begins to wear away, look closely to see the lines and shapes that are produced as the shell grew.


* What shapes and lines appear?
* How long did it take to begin to see through the shell? What does this reveal about the purposes and functions of the shell?

7. A Water Skin


* 2 drinking glasses
* Water
* Vegetable oil


Fill each glass with equal amounts of water. In one glass, put 2 tablespoons of oil. Set the glasses in a place where they will be undisturbed but easy to see. Observe the glasses over the next few days.


* What happened when the oil was added to the water?
* What happened to the water in the two glasses over the course of a few days? Which one had more water? Why?


Choose and/or modify words or definitions to fit your needs and grade level.

Cuticle - a general word describing the protective, noncellular, organic layer covering many invertebrates, plants, and even sometimes used to describe the epidermis of higher animals.
Dermis - the inner, sensitive, vascular layer of skin.
Epidermis - the outer, nonvascular layer of skin.
Exoskeleton - a hard, protective covering found on insects, spiders, and other arthropods. The outer "skeleton."
Membrane - a very thin, flexible sheet, usually made up of two layers of fat molecules. Membranes surround all cells.
Osmosis - water traveling through cells in order o equalize the concentration of salts, sugars, etc. on either side of a cell membrane. Water travels from an area of less concentration to an area of high concentration thus diluting the more concentrated solution.
Ratio - the relationship (usually mathematical) in quantity, amount or size between two or more things.
Skin - in the case of humans, a living organ on the exterior of the body comprised of the epidermis, dermis and subcutis layers.
Stoma - a tiny pore on the surface of a plant's leaves and stems that allows gases to pass into and out of the plant. They release oxygen and absorb carbon dioxide. (Plural=stomata)
Subcutis - the deepest layer of skin, highly vascularized and the origin of hair follicles.
Surface Area - length x length.
Transpiration - the process by which a plant draws water (usually from the soil) up from its roots to its leaves. Also describes the process by which a plant loses water through stomata on its leaves and stems.
Volume - length x length x length.

Resources for "Keeping the Insides In:"

Allison, L. Blood and Guts: A Working Guide to Your Own Insides.
Arthur, A. (1989) Shell. NY: Alfred E. Knopf.(This is an Eyewitness Book with wonderful photographs and lots of good basic information on shells.
Burnie, D. (1991) How Nature Works. NY: Reader's Digest Association, Inc. (A good, general book about nature with lots of experiments and great pictures. Some activities relate directly to skins, etc.)
Burton, M. & R. (1975) Encyclopedia of Insects and Arachnids. NY: Crescent Books. (A book about insects and arachnids in encyclopedia form. Lots of good photographs.)
Duden, J. (1990) The Ozone Layer. NY: CrestwoodHouse.
Exploratorium Quarterly: Edges (Fall 1987. Volume 11, Issue 3) This collection of articles extends the subject in interesting directions and has suggestions of things to do and notice.
Kindersley, D. The Visual Dictionary of the Human Body.
Ranger Rick's Nature Scope: Discovering Deserts. (1986) (Nature Scope's always have excellent activities. This one devotes some time to desert plants' special, protective coverings.
Snodgrass, R.E. (1967) Insects: Their Ways and Means of Living. NY: Dover Publications, Inc. (A more technical, yet readable, book on insects.)
The Human Body: A Comprehensive Guide to the Structures and Functions of the Human Body. (1989) NY: Arch Cape Press. (An introduction to the human body, including the skin, with excellent diagrams and written in laymen's terms.)
General Resources:
Hann, J. (1991) How Science Works. Pleasantville, NY: Reader's Digest Association, Inc. (A great resource for all science topics. Good illustrations and photography.)
Strongin, H. (1991) Science on a Shoestring. Menlo Pk., CA: Addison-Wesley Publishing Co. (Another good general science book.)
DeVito, A. (1989) Creative Wellsprings for Science Teaching. W. Lafayette, IN: Creative Ventures, Inc. (Good general science book.)
Science For Children: Resources for Teachers. (1988) National Science Resources Center. Washington, D.C: National Academy Press. (Fantastic sourcebook for teachers containing curriculum, book and location resource references.)
An Explorit Science Center "Science Byte":Keeping the Insides In.

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