12 months old
The joke The cat meowed.
Why it's giggle-worthy You had to be there. As babies get older, they rely less on watching adults to know what's funny; they begin to make the call on their own. Odd sounds are particularly hilarious, as evidenced by a few popular YouTube videos that show babies laughing hysterically as Dad makes strange, high-pitched pinging noises or tears a piece of paper. (Search YouTube for "laughing baby" if you need a pick-me-up.) When Jessica Blaser's daughter, Davis, was 5 months old, the mom from Canyon, Texas, could make her laugh just by saying "knock, knock." "She had no idea what I meant, but for some reason, she cracked up every time." Experts aren't sure why certain words and sounds are comedy gold, but hey, whatever works!