Number 18: The bottle rocket

Rocket science isn't exactly brain surgery. A few odds and ends and a bicycle pump is all you need to build a pretty impressive rocket.

Safety first:
The rocket can take off with quite a bit of speed.

• Make sure there is a responsible adult present.
• Don't point the rocket towards any people or animals.
• When you launch the rocket make sure it's pointing away from you.
• Launch the rocket where there is plenty of space for it to come down. A playing field is ideal.

You'll need:

• An old bicycle inner tube.
• A cordless drill.
• An empty soda bottle.
• A cork or bung that fits snuggly into the bottle.
• Sticky tape.
• Drinking straws.
• 4 wooden skewers.
• A bit of thin card.
• A pump (a foot pump or track pump will do nicely).
• Scissors

How to build it:

1. Cut the valve out of the inner tube. Trim off the excess tubing from around the valve.

2. Drill a hole through the cork.

3. Push the valve through the hole in the cork. It needs to be a tight fit, so you might have to tap it in with a mallet.

4. Cut the drinking straw into 4 bits. Fold over one end of each bit of straw. Tape them to the bottle with the open end pointing towards the neck of the bottle. 

5. Make a nose cone from the card and tape it to the bottom of the bottle.

6. Pour water into the bottle until it's about 1/4 full.

7. Push the cork into the bottle.

8. Attach the pump to the valve.

9. Stick the skewers into the ground and slide the straws onto them  so that the rocket is resting on the skewers.

10. Make sure the rocket is pointing away from you then start the countdown and get pumping!

These are 3 successive frames from a movie shot at 30 frames per second. So you can see the rocket launches pretty quickly!

What's going on?

The physics involved here is really quite simple. The same principles apply as with the film canister rocket. In that case we had a container that filled up with gas as a result of a chemical reaction. With the bottle rocket the pressure build up is caused by you pumping air in. Eventually the pressure gets too great and the cork pops out. The air pressure then forces the water out, the action of the water moving down and out of the bottle in turn propels the bottle upwards.

It's another great example of Newton's 3rd law of motion which states: "To every action there is an equal and opposite reaction". So in this case the action is the water being forced out of the bottle, and the reaction is the rocket shooting off.

Number 17: The Stopped Clock Illusion

Our brains are really astonishing things. There's a whole lot of stuff that we think we see, but really our brains are just making stuff up to fill in the gaps in our perception. But sometimes we can catch out brains out and see through the illusions they create for us. One example is with the blind spot on the backs of our eyes that I described it a while ago. Here's another example called 'the stopped clock illusion'. Maybe you've noticed before when you first look at a clock.

You'll need:
A clock with a second hand.

What to do:
Its really simple. Just look at the second hand on the clock. Look away, then quickly look back.

The hand appears to stop for longer than a second. Then it ticks on around the clock face!  Weird isn't it?

What's going on?
The technical name for this illusion is chronostatis (from the Greek chronos meaning 'time' and stasis meaning 'standing').

Scientist aren't completely sure what causes chronostatis. One theory is that our brains can't process information from our eyes whilst they are moving rapidly. So our brain 'backfills time' with whatever the eyes see after they've stopped moving.

If you want to know more here's a (reasonably) easy article to follow.