I’m sure you’ve seen these things around. I have one, actually–a tiny gray alien that grows many times its size when placed in a glass of water. When you take it out and let it dry, it goes back to its original tiny size.
Here are a few:
http://buyriteinc.com/br_gallery.asp?page=5&subCatId=13&CatId=12&search=&hotItem=
What do they make these out of? By what chemical reactions do they grow in water and shrink in air?
I’m not sure what compound they’re made from (some sort of rubber polymer or something), but the expansion is due simply to water absorption. It might help think that instead of “expanding 600%” it just means that in their fully “grown” state, they are made of 60% water – or if you prefer their original miniature state they have had just about all of the water removed.
The same sort of thing can be observed in those dollar store packaged rags (they’re about 1" square by half an inch thick but expand in water to about the size of a normal facecloth) or the those little packets of gel crystals you pour in a tealight candle bowl and add water. They start out pretty tiny (a few grains of sand tiny) and expand to 10-20x their size in water (though as you can imagine, by that time they’re pretty soggy, much like Jell-o)
Thanks. I’ve been curious about this process for a while.
Nitpick - approximately 85.7% water by volume is more like it. Assuming they start off totally dehydrated, then expanding 600% implies they they become 1 part whatever and 6 parts water, i.e. 6/7ths water by volume.
Sorry – math moron here. 
The principle stands though.
Science moron here: What is a polymer?
A polymer is a mixture of two or more compounds combined together by a process called polymerization, which is, in basic terms, the molecular restructuring compounds to form larger combined molecule chains that contain repeating structural units.
In case that still doesn’t make sense, plastics, rubbers and synthetic fabrics are all polymers. And some paints.
Another math moron calling in.
I thought 600% expansion meant 6 X expansion*. So 1/6 hydrophilic polymer and 5/6 water.
Anyways, shouldn’t % be reserved for fractions?
50% --> 1/2 X
100% --> 1 X
200% --> 2 X
600% --> 6 X
There are two ways percentage growth is stated:
“An increase of 100%” means a doubling in size. The increase is 100% as big as the original. This is presumably what was meant by a 600% expansion so the increase was 6 times the original so it’s 7 times as large now. This is what is done with sales taxes for example. A 6% sales tax is the increase over the price.
The other was is something like “Increased to 500% of its original size” This means it’s now 5 times as big so it was an 400% increase.