DES MOINES, Iowa (AP) — There’s no trick to earning a treat for Halloween in Des Moines. All that’s required is a good joke.

Even if it’s a bad joke.

“What’s the skeleton’s favorite instrument? The trombone!” said 10-year-old Anna Mattern. “But you have to emphasize the bone.”

For decades, as the city’s make-believe ghosts and goblins trekked door to door on Beggar’s Night, or the night before Halloween, they’ve had to perform to earn their treats.

The custom began at the suggestion of a former city parks director as a way to tamp down on Halloween vandalism and emphasize wholesome childhood fun. The initial idea was to have children sing a song, recite poetry or offer other kinds of entertainment, but over the years the tradition of telling a joke took hold.

The idea stuck and has spread from Des Moines to most of its suburbs and a scattering of cities throughout the state.

“I usually have one joke I tell at each house,” Mattern said this week as she shopped for a costume with her mother and brother in West Des Moines.

Lesley Pratt Bannatyne, an author and expert on Halloween customs, said other communities occasionally celebrate Halloween on other days, but it’s unusual to consistently hold the event a day early. And she’s never heard of requiring kids to tell a joke.

“It’s fascinating that people go along with it,” said Bannatyne, author of the 1990 book “Halloween: An American Holiday, an American History” and editor of “A Halloween Reader,” a compilation of stories and poems from the past 400 years.

While telling jokes for candy seems unusual, Bannatyne said it hews closer to old European traditions in which people would dress in a disguise and perform in return for food, candy or drinks. She welcomed anything to slow down what’s usually a mad dash for candy and make it more personal.

“It’s not meant to be extortion,” she said. “It’s meant to be an exchange.”

The city no longer promotes its Halloween alternative, but Des Moines City Clerk Diane Rauh, a lifelong resident of central Iowa, said for as long as she can remember Beggar’s Night has been on Oct. 30, the only night kids trick-or-treat in Des Moines.

“Not having lived anywhere else, I’m not sure what the traditions are elsewhere. Do you just ring the doorbell and they give you candy?” Rauh asked. “We make you work for it in central Iowa.”

She said the practice is especially confusing to new residents.

“They just kind of look at you,” Rauh said.

Anna Karr, 9, of Des Moines, and her mother, Tamara Rood, were getting ready this week for Beggar’s Night by shopping for a costume. Anna hadn’t picked a joke yet, but figured she would look one up in an encyclopedia for jokes.

Her mother, who grew up in Ames, north of Des Moines, said she remembers telling jokes for candy when she was a child.

“I always thought it was fun,” Rood said, “and I don’t think it’s too much to ask.”