We had some capsules that when you put them in water, "grow" an animal. We had 4 types and we've done 2 of them: Sea and Zoo/Jungle:
Here are the little capsules.
In a bowl of water.
The capsules weren't dissolving very well, so we rubbed the foam animals to get the gel off of them faster.
Playing with the animals
Because the bowl idea didn't work very well, we decided to put them in bottles of warm water and shake them.