I just looked over your science snack page and am impressed. It does a really nice job of describing creative ways to make science understandable. Here is a new idea for a snack.
Title - Shrinking rubber band is a "Cool" idea
Materials - One fat rubber band
Hold the rubberband so that it hooks around the thumb for each hand. Stretch the rubber band out (don't snap yourself!) and wait a few (5 to 10) seconds. Quickly bring your hands together so the rubber band gets smaller. Once the rubber band is back to it's original size, quickly touch it to the skin between your upper lip and nose. What do you notice? Does it feel cool? Let the rubber band sit for a little while and touch it to your lip again. The rubber band doesn't feel cool any more.
What's Going On?
The skin between your lip and nose is very sensitive to small changes in temperature. When you stretch the rubber band, you are doing work. As the rubber band gets tighter, the molecules in the band rubber band are squeezed. This makes them hot (the same amount of heat energy from before is now concentrated in a smaller space. This makes the item hotter). When you hold it for a few seconds, the air around the rubber band cools the rubber back to room temperature. When you release the rubber band quickly, the rubber gets a little bit bigger (takes up more space for the same amount of rubber). Since the heat energy in the band is the same and it must now occupy more space, the temperature of the rubber band drops a bit until it can absorb heat from the air around it. If you touch it to your lip just after it shrinks, you can feel it get cooler.
What else can you do?
You can touch the rubber band to your lip just after you stretch it. Make sure to stretch it quickly. Now, you can feel the rubber band is warm. This is the heat energy that is squeezed into a smaller space and makes the rubber feel warm.
Well, that's about it. A pretty simple experiment that anybody can do. I think you would have to be 8 or 9 to understand the science behind it, but even a 4 year old can do the experiment over and over. About the only safety precaution is being careful not to snap yourself or stretch so far that the rubber band breaks.
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