18 months old
The joke Making a mistake--on purpose
Why it's giggle-worthy At 18 months, your little funster has the world (or at least his portion of it) largely figured out, so nothing is more hilarious than making a blunder--intentionally--or pointing out when someone else makes one. "When toddlers make a deliberate mistake--for instance, singing the wrong words for a song they know--they're demonstrating an awareness of cognitive incongruity," Dr. Bergen explains. The joke is that they know what's supposed to happen, but it doesn't, like when Mr. Noodle on Sesame Street uses a banana as a trumpet. Carrie Wallace, of Lawrence, Kansas, experienced this with her son: "My 18-month-old, Dmitri, has animal toys that came with matching hats. He'd put the wrong hat on one, look at me, then laugh uproariously." With your would-be Will Ferrell's budding language skills, he just might tell you that a cow says "baa," or that Mom's name is Daddy. Overturning the social order? That's a scream.