How do you fancy making a stunt bike? They're really easy to put together and can be used for all kinds of fun challenges!
Here you can see a stunt bike jumping over six double decker buses!
They're really powerful, because they amplify the force using something called a flywheel.
If you'd like to make one, you'll need two free CDs (the kind you get in magazines and through the post - don't use your latest music CDs)...
...two cotton reels...
...two ballpoint pens with lids...
...some strong glue...
...and a piece of string that's about the same length as an arm.
The first job is to glue the two CDs together.
Do this so that the shiny sides face outwards, and make sure that the holes in the middle line up perfectly.
Next, you need to glue a cotton reel on either side of the two CDs.
You need strong glue for this. Kate used a glue gun, but you can use any strong tubed glue.
Always get help when using any strong glue - it can get very fiddly!
Attach a cotton reel on either side. Again, make sure that the holes line up perfectly.
The axle is made from one of the ballpoint pens.
Remove the lid, and also the inside of the pen.
You don't need the inside, so you can throw it away.
Pop the lid of the pen back on...
...and thread the barrel of then pen through the wheel.
When done, pop the lid of the second pen on the end, to keep it in place.
You could glue the lids in place, but it's not essential.
The axle you've made should now allow the wheel to spin freely.
Now grab the string, and wrap it around one of the cotton reels.
In a real motorbike, the energy is stored in the petrol.
But in these stunt bikes, the energy is stored in the freely-spinning CD flywheel.
The stunt bike uses a fixed-gear mechanism.
Give it a turn and you'll see that one turn of...