It's hard to watch a boomerang soar off through the air and return to the thrower without imagining that some kind of magic is at work. In fact, it's not magic at all -- just physics taking flight.
Depending on whether you are left-handed or right-handed, mark the cardboard square as specified and then cut out the boomerang. It will resemble a chunky X at this stage. Use the scissors to round the ends for safety. Use this illustration for right-handed throwers:
For left-handed throwers:
With the ballpoint pen, score along the fold lines (shown as dashes in the diagram) and then fold the cardboard down to create 45-degree flaps. If you like, decorate the boomerang with stickers or colored markers.
Try out your boomerang on a calm day, standing 20 feet away from people or obstacles (boomerangs are notorious for getting caught in trees or landing on roofs). Hold the boomerang nearly vertical (not horizontal, as you would a flying disk), with the pencil lines visible. Flick your wrist to add spin as you toss it forward and slightly upward. It should start to turn and then flatten out as it coasts back to you.
The easiest way to catch it is to clap it between both hands. Don't be surprised if it takes you a few tries.
What's Happening: There's a lot of physics going on here, with your boomerang acting like a set of wings and a spinning gyroscope.
For starters, by bending back the edges of the boomerang arms, you turn them into curved wings, like those of an airplane. The curves cause air to move more quickly over the wings' tops than the bottoms, creating lift. That's what keeps the boomerang from falling to the ground.
The reason the boomerang comes back to you has to do with its spinning motion, similar to the way a gyroscope or top spins in a circle. If you're interested in the details of how this works, we like the explanation at HowStuffWorks.