If that doesn't make you chuckle, it's not because you're too dense to get a good joke. It's because a 4-year-old made it up. He thought it was hysterical.
Humor is an intensely personal experience, but for young children it is also developmental. What they experience as funny depends on their cognitive understanding and their ability to distort what they know. The typical 4-year-old can't grasp double meanings or word play, but he can mimic the pattern of knock-knock jokes. That alone can make him laugh.
Researchers may marvel at this ''pre-riddle" stage but parents mostly bemoan it. Who wants to hear the same un-funny joke repeated and repeated? Laugh anyway. If nothing else, it's important to encourage children to try again.
''Humor in children has been correlated with higher intelligence, creativity, sociability, empathy, self-esteem, and problem solving," says psychologist and humor researcher Louis Franzini of San Diego State University. Children with a good sense of humor tend to be well-liked by peers and by adults. Franzini, author of ''Kids Who Laugh, How to Develop Your Child's Sense of Humor" (Square One Press), calls laughter ''the legal high."
There's long been speculation about why human beings like to laugh, but it was only in 2003 that researchers discovered neurological evidence that the brain is wired to take pleasure from humor and laughter. That's good news for parents. It means that for your child to have a good sense of humor, the most important thing you can do is nurture and support it.
Pediatrician Mark Waltzman worked as a clown during college and uses humor to defuse children's fear at Children's Hospital, where he works in the emergency room. ''It has to be appropriate to the situation and the child, of course," he says, ''but if you can make a patient relax and be calm, everything goes more smoothly." With a young child, he'll often start by vigorously washing his hands and making bubbles with the soap. With a school-age child, he may begin an exam by asking, ''Are you married?" He's building on children's delight in incongruity. ''Having an adult ask such a silly question in such a serious place cracks them up," he says.
One of the nice things about laughter is that it can make you feel good even when you're by yourself. But it's even better when you make someone else laugh.
''There's a bonding process that occurs. You feel closer to someone you share a laugh with. That can happen as young as 12 months," says Paul McGhee, one of the nation's preeminent researchers on children's humor. A professional speaker, he is author of ''Understanding and Promoting the Development of Children's Humor" (Kendall/Hunt) and president of laughterremedy.com.
Don't confuse a baby's first laugh with humor, though. An early laugh is usually in response to a physical sensation such as tickling. ''It reflects pleasure but not humor. There's no thought process," says McGhee.
It's typically between 6 and 12 months that a baby begins to delight in a parent's or caregiver's unexpected actions. McGhee's favorite example is of a 7-month-old who laughed out loud because mom sucked on his bottle. The mother wasn't trying to be funny, she was checking to see if the nipple was clogged, but it violated the baby's understanding of what is supposed to be: babies suck on bottles, not mommies!
Without realizing it, most parents signal that something funny is about to happen. ''We have exaggerated ways of talking, or extreme facial expressions that say: 'This is silliness,' " says educational psychologist and humor researcher Doris Bergen, of Miami University of Ohio.
Some months after they react to incongruity, toddlers initiate it, first with clowning actions and then, at about 2 years, with words -- for instance, calling objects by the wrong name or substituting nonsense words into a song.
Twenty-five-month-old Lauren Geise of Arlington has been loving a game like this for six months, ever since parents Shannon and Jay first asked her, ''Who are you? Are you Elmo? Nooo! Are you Ernie? Noooo! Are you mommy? Noooooo! You are. . . Lauren!" Now she initiates the game. ''It's such a source of pleasure that we can use it as a diversion when she's on the verge of a meltdown," says Shannon.
As their understanding of language deepens at 4 and 5, Bergen says, their humor more and more reflects multiple meanings in words: covering the dog with a blanket and calling him a hot dog; telling a ''tall tale" about growing as tall as the ceiling and jumping over the house. From there, riddles and knock-knock jokes become the perfect showcase for the new-found skill with words.
At older ages, humor becomes more individualized and patterns are too complicated to predict. The best response from a parent at any stage is to share in a child's delight. Bergen says that if you ask a young child why something is funny, even when they don't understand the word play, the answer often is, ''Because it makes my mom or dad laugh."
''To want to please, to mutually enjoy, those are good things," she says. Bergen also urges adults to use humor as a way to teach resilience.
Rob Schneider and Laura Tully of Lexington have caught on to that with their son, Simon, 10. ''Little things can fluster him. He'll fly apart," says Schneider. On the other hand, he has a great sense of humor. At the movies recently, he wanted bite-size Snickers candies called Poppables. When he opened the bag, they popped out, all over the ground. Simon was on the verge of tears when Tully quipped, ''I guess that's why they call them Poppables!" Instead of crying, Simon laughed.
Since then, the incident has been a catch phrase in the family when Simon is on the verge of making too big a deal out of something.
Of course, there are times when parents and teachers understandably get annoyed or frustrated at silliness. Who's to say when one classmate's nonsense rhyme about boots (boots, hoots, moots, coots) will disrupt the entire class, or when one sibling's silliness about peas escalates into gales of laughter about pees and ruins dinner.
What parents find offensive in potty humor is what delights children. ''There's shock value," says Bergen. ''It breaks the boundaries of what they can say and do." The more we react and the stronger our reaction, the more delight they take.
Franzini frowns on punishing children for expressing humor, but that doesn't mean anything goes. As with any other inappropriate behavior, limits and consequences need to be clearly established. For instance, ''In our family, we don't make jokes that are cruel or hurt people's feelings." With potty humor, Franzini recommends saying, ''That may be funny with your friends, but it's not funny to adults."
It's not unusual for a riddle your child tells you to be one you loved at the same age. They have a habit of recycling, says Bergen. Keep that to yourself, though. When you tire of the same riddle, the best defense is a good offense. See if your child has heard any of these, which Bergen says are circulating today:
What days are the strongest? Saturday and Sunday. The rest are ''weak" days.
Who are the most overweight superheroes? Fat Man and Blobbin.
Why did the skeleton cross the road? No guts.
Why did the elephant take toilet paper to the party? Because he's a party pooper.
Contact Barbara Meltz at email@example.com.