What to do

1. Firstly, put one of your cups inside the other, this will give you an idea of the size of catch you will need. Next bend your paperclip into the shape shown in the picture. The hook at the end should be quite small and at the right angle to keep it secure, but still slip off easily when triggered. The next bend needs to be at just the right position and angle so that it fits over the lip of the lower cup and will act as a pivot when the catch moves. Finally, the long end of the paperclip has to be at a level lower than the rim of the lower cup. If it doesn’t stick out enough it may fail to trigger. If it sticks out too much then a lot of the elastic band’s stored energy will go into throwing the bottom cup downwards at the table – you’ll get a lot of noise but the top cup won’t jump as high. You’ll need to experiment with bending and re-bending the paper clip until it sticks out by just the right amount for your cups and elastic band.

2. You’re now ready to charge and set your machine. Wrap the elastic band around one of the cups and then carefully push the other cup inside it. DO NOT LET GO!

Fit the trigger as shown and the two cups should be held together.

3. To trigger the catch, simply hold the two cups, rim downwards, and drop onto a firm surface. The long end of the clip is pushed into the cup by the surface, the whole clip pivots about the rim of the lower cup, and the hook slips off the rim of the upper cup allowing the elastic band to fire it into the air.

Other things to try

• To replace the trigger with a time-delay mechanism, try using small rolls of sticky tape on the top of the bottom cup to stick to the top cup – eventually it will give way and let the top cup fly up.
• Try with more cups stacked inside each other (you will need to re-work the catch)
• Or try with something a bit bigger than cups, like buckets!

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