The grown-up on the lime-green bike, decked out with matching green wheel disks, hollers, “Three, two, one…POLO,” and two groups of three kids, facing each other across the pavement, wobble forth on their bikes. Why are they wobbling? Because they’re also carrying long mallets. They’ll use these mallets to maneuver a small ball toward goals made of orange pylons set one bike width apart.

The kids are playing bicycle polo for the first time, led by bike polo enthusiast Matt Messenger, the guy on the lime-green bike. This is day three of Cascade’s Urban Riders summer camp for ten- to thirteen-year-olds. After learning the basics of riding in traffic on Monday and whizzing around the Marymoor Park velodrome on Tuesday, the campers are now getting coached in the rules and requisite skills of polo.

They’ve learned, for example, that a hardcourt bike polo court is about the size of a tennis court and that there are two ways of using the handmade mallets, which are shaped like croquet mallets. You can shuffle the ball down the court with the wide end of the stick or shoot using the smaller surface area at the front of the mallet. You have to shoot to score. If you make a goal using the wide end of your mallet, I’m sorry—it doesn’t count.

Games are played three against three and up to five points. Put your foot on the ground during the course of play, and you’ve dabbed—as penalty you must ride a full 360 degree turn before handling the ball again.

Matt offers ample time for practice and then gets a game going, with kids rotating in and out of play. There’s a lot of toppling and crashing, lots of fumbling to get the ball. Cascade volunteer William Gerdes, on hand all week to supervise campers, says, “The most dangerous thing is when you have the ball and get excited. You go fast and lose track of where the other bikes are.” When asked about the hardest part of the game, 12-year-old Justin says, “To score!”

After a while, Matt calls out, “That was a REAL game of polo!” It’s time to break for lunch.

During the meal, Matt talks more about the game. He tells me that Seattle was the birthplace of hardcourt bike polo (as opposed to traditional, grass-field polo, developed in Ireland in 1891). About 35 people currently play in Seattle, and a team from the city placed fourth in the National Championships in Madison, Wisconsin this year. Adult games are intense, he says, with speeds up to 35 miles per hour, and skids starting from 20 miles per hour. Watch this video to see hardcourt bike polo in action.

That’s harder-core, hard-court playing than I witness from the kids, back on the pavement again after lunch. And yet their skills are definitely improving. I watch Nico, 13, shuffle the ball across the court at high speed and then adjust his mallet to score with the small end. The day’s MVP is ten-year-old Steven, also the smallest player of the bunch. Surprised to learn that he’s made eight goals that afternoon, he says, “Too bad we have to leave. That was fun!”

The day ends with a group photo and a chant: “Bike polo, bike polo, bike polo!” It sounds like most of the kids agree with Steven—bike polo is fun!