Humor has always been subjective. Where some people prefer the physical comedy of Jim Carrey, others may find the dry approach of Albert Brooks preferable. Babies, of course, are not big on subtlety. Making silly faces, tickling their feet, or pretending you've disappeared in an endless loop of temporal displacement-otherwise known as peek-a-boo-are all ways to get tiny people to laugh. But why exactly are they laughing? Is it because they find their parents funny, or it is a reflex?