The Comeback Can is one of my favorite trick robots. It looks like an ordinary tin coffee can decorated with colorful paint. I like to invite people to watch as I roll the can away from me. In a few seconds the can slows down, then mysteriously returns. Everyone asks what the secret is. I tell them that the can loves me and that it gets nervous when it strays too far. But I'll let you in on the real secret: there's a rubber-band powered motor inside the can. It's easy to make.

SAFETY KID SAYS: "Because this experiment requires holes to be punched or drilled into a metal can, make sure a grown-up is around to help!"

Metal coffee can with lid, or any other large cylindrical can with a sturdy lid
Paper clip
1 heavy nut (the kind a bolt goes in), or a rock the size of an egg yolk
Long rubber band
2 toothpicks

Acrylic paints
Construction paper
School glue

Drill or screwdriver
Tape
Paintbrush

1   With the drill or screwdriver, make one small hole in the center of the bottom of the coffee can, and another in the center of the lid.

2   Unbend the paper clip and twist it around the nut as shown in Figure 1. If you're using a rock, tape it to the paper clip. Then attach the paper clip to the center of the rubber band.

3   Thread the rubber band through the holes in the can and lid and stick toothpicks through the exposed loops, as shown in Figure 1. Tape them down.

4   Test the can by rolling it. If it doesn't come back to you, the rubber band might be too loose or too tight. Also, make sure the weight isn't touching the side of the can.

5   Once you're happy with the can's behavior, decorate it with paint and construction paper. Use the paper to hide the toothpicks and rubber band.

Figure 1

HOW IT WORKS: As you roll the can away from you, the weight causes the rubber band to wind up. This is like winding up a spring-powered toy. When the rubber band starts to unwind, the energy it releases causes the can to return to you.

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