Another pinner says: How Many Drops of Water Can a Penny Hold? My daughter did this as her science fair project last year and it was very popular. She even used water variations (Kool-aid, salt water, etc).
Here is a great science craft to teach kids about alternative energy: a solar oven! Harnessing the power of the sun is cool and teaches us about caring for our environment and using renewable resources. Best of all, kids can make a snack they love, nachos