I think this is the earliest thing babies respond too. There's nothing like hearing the baby laugh when you play peek a boo with them. My boy just giggles and grins the whole time.

That is cute. What my son does when kids are a little older, is ask them to give them a high five, and when they slap his hand, he just screams owww and fakes crying. Most of the time the children laugh and do it again.

Surprising how little it takes to amuse a child isn't it? Too bad we have to get older and have it take so much more just to make us smile.