I used to have these all the time.
My favourite by far was this egg-like sea monkey-- Some kind of reptile was supposed to 'hatch' from it when you put it in water. I got it at a random convience store when I was nine or so, and put it in a glass of water when I go home. Over a couple days, the sponge-y snake inside got big enough to break the shell and lo-and-behold, it 'hatched.'
I kept the snake until it mysteriously disappeared one day. xD
~Mew