Note: Do the pendulum experiment first, and when you’re done with the heavy nut from that activity, just use it in this experiment.

You can easily create one of these mystery toys out of an old baking powder can, a heavy rock, two paper clips, and a rubber band (at least 3″ x 1/4″).  It will keep small kids and cats busy for hours.

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Here’s what you get:

  • can with a lid
  • heavy rock or large nut
  • two paper clips
  • rubber band

You’ll need two holds punched through your container  – one in the lid and the bottom. Thread your rubber band through the heavy washer and tie it off (this is important!).  Poke the ends of the rubber band through one of the holes and catch it on the other side with a paper clip.  (Just push a paper clip partway through so the rubber band doesn’t slip back through the hole.)  Do this for both sides, and make sure that your rubber band is a pulled mildly-tight inside the can.  You want the hexnut to dangle in the center of the can without touching the sides of the container.

Download Student Worksheet & Exercises

Now for the fun part… gently roll the can on a smooth floor away from you.  The can should roll, slow down, stop, and return to  you!  If it doesn’t, check the rubber band tightness inside the can.

The hexnut is a weight that twists up the rubber band as the can rolls around it.  The kinetic energy (the rolling motion of the can) transforms into potential (elastic) energy stored in the rubber band the free side twists around. The can stops (this is the point of highest potential energy) and returns to you (potential energy is being transformed into kinetic). The farther the toy is rolled the more elastic potential energy it stores.


  1. Explain in your own words two types of energy transfer:
  2. True or false: All energy in a system is lost to heat.
    1. True
    2. False


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2 Responses to “Mystery Toy”

  1. It’s hard to say without seeing your project. Can you be more specific, and let me know what happened (or didn’t), and send me a photo/video of the inside?

    Part of being a great scientists is learning how to observe things carefully, especially when things happen that you don’t expect (or don’t happen that you do). Most people give up, when really it was only a quick fix or adjustment that needed to be made.

  2. When I did my project it did’t work.Do you now what’s wrong?