Every so often a paper toy gets the ratio exactly right: thirty seconds of folding, several minutes of delight. The roto-copter is one of them. Cut three slits, fold four times, add a paper clip, and you have a helicopter that spins its way down instead of fluttering.

Two details decide whether yours flies well. First, the blades must curl in opposite directions — one toward you, one away — because that twist is what catches the air and sets up the spin. Second, the paper clip on the stem is not optional; the weight keeps the descent steady and stops the toy tumbling. If your first attempt wobbles, check that the blades are bent firmly rather than barely.

Ordinary printer paper is ideal, and there is plenty to experiment with once you have the basic shape: shorter blades spin faster, a longer stem descends farther, and a second paper clip changes the flight entirely. Open a window, climb the stairs, release, and repeat until someone else wants a turn.

STEP 1. Take a rectangular sheet of paper and cut three slits into it as shown.

STEP 2. Fold the lower-right section over to the left.

STEP 3. Fold the lower-left section over to the right.

STEP 4. Fold the bottom edge up so those two flaps stay in place.

STEP 5. Clip a paper clip onto the bottom edge to weight the stem.

STEP 6. Slightly bend the top-right blade toward you.

STEP 7. Slightly bend the top-left blade away from you, in the opposite direction.

STEP 8. Your Roto-Copter is ready! Hold it by the weighted bottom, drop it from a height, and watch the blades spin it all the way to the ground.