This can entertain the little ones at Easter if you put on your wizard hat and ham it up enuf.
Present them with two eggs, one hard-boiled and one raw. Ask them to pick the cooked one (no fair breaking shells!) After they've done everything they can think of and given up, do the following.
Spin one and then gently tap it. Do the same to the other. One will spin rapidly, and stop immediatly when tapped. The other will spin slowly in a wobbly way, and after being tapped, will continue to spin. That one, of course, is the one that's liquid inside.