Two super easy experiments to do when studying osmosis and dehydration.
Shrink an egg (diffusion through a membrance is called osmosis):
Boil an egg, remove shell and place in zip lock bag filled with clear corn syrup. Check it every couple of hours the first day. You'll see it shrinking as the water in the egg passes through the egg's membrance into the corn syrup. Leave it sitting a whole week before observing again. Ours shrunk down to the size of the yoke.
Dehydration and evaporation (the loss of water from an object): 
Take an apple and peel it. Using a spoon or a butter knife, carve a face into the apple as you would a jack-o-lantern. As the water leaves the apple, it will shrivel up to look like an old wrinkled face.
Cay, the egg one sounds messy ... a definite plus for doing with my sons!
The apple one is a great one for this time of year -- one of those cross-curriculum type of experiments that can be fun too; we've made some really cool old hags this way but never thought to explain the "science" behind it!
Posted by: Mary G | October 24, 2006 at 08:23 AM
Those are great ideas, Cay! I think we may just have to keep those in mind!
Posted by: Theresa | October 24, 2006 at 08:38 AM
Fabulous ideas. I love that they require so few materials. I have so much trouble gathering things.
Posted by: Jennifer | October 24, 2006 at 08:42 AM
The egg one sounds perfect for us! I love science experiments that are contained in a baggie or bowl.
Posted by: Jennifer | October 25, 2006 at 12:40 PM