Slinging Machines

Every Halloween, students at a school in New Hampshire build medieval-style weapons for an epic water balloon fight.

October 1, 2026

By Jennifer Barone

Trebuchet Day

Each year, Windham High School in New Hampshire holds its annual trebuchet day. Watch one team of students test their homemade weapon.

Halloween is a special time at Windham High School in New Hampshire. Students get to pelt their teachers with water balloons—using machines the teens built themselves! The yearly tradition is known as Trebuchet Day.

During the Middle Ages, European military forces used trebuchets [treh-byuh-SHAYS] to attack castles. These weapons could launch a projectile, like a boulder or pot of flaming material, hundreds of yards. Inspired by these ancient machines, physics teacher Pat Kaplo challenges his students to explore projectile motion by designing water-balloon-launching trebuchets. Teams have about a month to build them. “Mr. Kaplo gives some tips, but mainly we’re figuring it out as we go,” says student Ethan. “You might go through many versions of your design,” adds Nick, another student. “At first it’s not going to work the way you think.”

Halloween is a special time at Windham High School in New Hampshire. Students use machines to pelt their teachers with water balloons. And the teens built the machines themselves! The yearly tradition is known as Trebuchet Day.

During the Middle Ages, European military forces attacked castles with trebuchets [treh-byuh-SHAYS]. These weapons launched projectiles. They could send a boulder or pot of flaming material hundreds of yards. Physics teacher Pat Kaplo got an idea from these ancient machines. He wants his students to learn about projectile motion. So he challenges them to design water-balloon-launching trebuchets. Teams have about a month to build them. “Mr. Kaplo gives some tips, but mainly we’re figuring it out as we go,” says student Ethan. “You might go through many versions of your design,” adds Nick, another student. “At first it’s not going to work the way you think.”

On Halloween morning, “you see trebuchets strapped onto cars and trucks driving to school,” says classmate Aliya. Teams wear themed costumes. Students and community members gather to watch the fun.

Teams arrange their machines in a circle 35 meters (115 feet) from a plywood “castle” at the center. Students earn points for distance and accuracy—including striking the castle, landing within the walls, or hitting the teacher inside. Teams can improve their score after the event by doing calculations to analyze the trajectory, or path, of their balloons.

“When a balloon comes close, the crowd gasps,” says Kaplo, who attends in costume as a medieval knight. “When I get hit, the squeals of joy are fantastic!”

Halloween morning arrives. Then “you see trebuchets strapped onto cars and trucks driving to school,” says classmate Aliya. Teams wear themed costumes. Students and community members watch the fun.

Teams arrange their machines in a circle. They’re 35 meters (115 feet) from a plywood “castle” at the center. Students earn points for distance and accuracy. That includes striking the castle, landing within the walls, or hitting the teacher inside. After the event, teams can improve their score by doing calculations. They analyze their balloons’ trajectory, or path.

Kaplo attends in a medieval knight costume. “When a balloon comes close, the crowd gasps,” he says. “When I get hit, the squeals of joy are fantastic!”

HOW A TREBUCHET WORKS

Here’s how a trebuchet can fling projectiles super-long distances.

PRE-LAUNCH:

The trebuchet’s arm is lowered and held in place by a cord connected to a small metal rod. A heavy counterweight on the other end of the arm is suspended in the air, storing potential energy. A balloon is loaded into the sling attached to the arm.

PRE-LAUNCH:

LAUNCH:

Pulling the metal rod releases the cord holding the arm. The counterweight drops. Its potential energy changes into kinetic energy, or energy of motion.

LAUNCH:

Pulling the metal rod releases the cord holding the arm. The counterweight drops. Its potential energy changes into kinetic energy, or energy of motion.

IN FULL SWING:

As the counterweight drops, the arm swings upward around a metal axle, flinging the sling into the air. The length of this axle is important: If it’s too long, it can bend or break when the arm swings.

IN FULL SWING:

RELEASE:

As the arm rises, a loop at one end of the sling slips off a pin at the end of the arm. The sling opens and releases the balloon. To fire accurately, builders adjust how their pin is angled so the sling releases at just the right time.

RELEASE:

HOW A TREBUCHET WORKS

Here’s how a trebuchet can fling projectiles super-long distances.

PRE-LAUNCH:

LAUNCH:

Pulling the metal rod releases the cord holding the arm. The counterweight drops. Its potential energy changes into kinetic energy, or energy of motion.

LAUNCH:

Pulling the metal rod releases the cord holding the arm. The counterweight drops. Its potential energy changes into kinetic energy, or energy of motion.