3 months old
The joke Mom is playing pat-a-cake with me--and laughing up a storm.
Why it's giggle-worthy Your tot is copying you. (Don't worry: He's laughing with, not at, you.) Special brain cells called mirror neurons hard-wire your newborn to imitate your actions, so he'll unconsciously return your smile and, in a few weeks, mimic your bursts of laughter. "It's like going to a comedy club," explains Gina Mireault, Ph.D., a professor of psychology at Johnson State University, in Vermont. "You laugh because everyone else is laughing." But there's another reason this game never gets old: Mom isn't usually this goofy. "A lot of humor in children is based on their realization that something is out of the ordinary or surprising, what we would call incongruous," says Doris Bergen, Ph.D., distinguished professor of educational psychology at Miami University, in Oxford, Ohio. Take it from Erin Wimmer, of Cedar City, Utah, who plays games to get giggles out of her daughter: "Kaitlyn laughs when I sing a peekaboo song while covering and uncovering my eyes."