Peekaboo is a game played over the world, crossing language and cultural barriers. Why is it so universal? Perhaps because it's such a powerful learning tool.

We'll admit, peek-a-boo is best-loved by babies who can't really insist on playing any games.

NASA's Hubble Space Telescope caught Jupiter's moon, Ganymede, seemingly playing a game of peek-a-boo in this image in April 2007.