*** BASIC PROCEDURE AND EXPLANATION ***

[This is my version of what I say to explain this experiment. When you have done it a few times you are sure to come up with your own and better spiel!]

- Hold the beach ball in the air and ask the children what will happen if you let go. Most of them will say that it falls down so let go and show them that they are right.

- Ask them why it falls down and get them to talk a bit about forces (use 'pushes' and 'pulls' if they haven't heard the word 'force' before).

- Then get them to all sit down on the floor and try to encourage any adults to join in, as the next part can be just as difficult for the adults as it is for the children and they will all enjoy the challenge!

- Explain how you are going to try and hold a ping-pong ball on the end of a straw. Ask them if they think it will be hard to balance it on the end of the straw - very hard - and allow those who think it is easy to have a go - it can be done if you have very steady hands!

- Ask them if they know another way- which doesn't involve glue etc. Many will have seen this done before and will know what to do. A few will suggest sucking through the straw which is much more difficult, but still possible.

- Hand out a ping-pong ball and straw to each of the participants (children and adults!). Bend the end of the straw and point it upwards to the ceiling.

- Hold the ball about an inch above the bent end of the straw and start blowing through it. Once you have started blowing let go of the ball. First attempts usually fail but with some practice you should be able to keep the ball up. The ball should be quite stable and you may be able to push it slightly and keep it in the air stream.

- Once everyone has had a reasonable attempt at keeping the ping-pong ball up you need to explain the science behind it.

- The very simple idea is that the air piles into the bottom of the ball therefore pushing it up and stopping it from falling. However, it is more complicated than this. You need to discuss the width of the air stream.

- They should all agree that it was very hard to balance the ball on the straw without blowing, but why? Get them to think about the size of the ball and the size of the end of the straw. If it isn't obvious then use the beach ball instead of the ping-pong ball! The air stream isn't the same width as the straw, but goes around the ball. This can be demonstrated using the big blower and the beach ball.

- Get the children to stand around the blower, turn it on and get the beach ball to balance on the air stream. They should be able to feel the air on either side of the beach ball.

- Turn the blower off again, and then demonstrate that blowing between two pieces of paper held a couple of cm apart causes them to close together. Fast moving air tends to suck things (in this case the two pieces of paper) into it (it is at a lower pressure than stationary air - this is called the Bernoulli effect).

- If they want to know why that is a slightly unsatisfactory explanation it is that the only thing that makes air move is a difference in pressure - it will move from high to low pressure - so if the air is moving fast in place A and slowly in place B the only thing that can have slowed it down is place B being at a higher pressure -> place A must be at a lower pressure...

- Back to the ball on the straw: If the ball starts to fall off the straw, it stops being in the centre of the stream of air. The fast main stream of air is now to one side, so the fast air sucks the ball back over the straw.

- The best way to show this is to turn on the big blower again and get them to try and gently push the ball lout of the air stream, they should be able to feel it push back. If they push too hard the ball will be pushed completely out of the air stream and will fall down. If you then get them to work out which side the air is going fastest on, and therefore which direction the ball should be pulled they normally get it. This means that which ever way the ball starts to fall off the straw it will get pulled back, so it is stable.

- An alternative way to think about this experiment is with Newton's Third Law. When the ball moves out of the centre of the air stream, the air is travelling along one side of the ball, and follows the ball's surface. This means that the ball is pulling the air away from the centre of the air stream, and so the air pulls the ball inwards towards the centre, making it return to the middle of the air stream every time it is slightly displaced.

- To demonstrate Newton's Third Law, you could think about throwing a heavy object - you get pushed backwards. Or you could ask if a wall pushes against your hand when you push it. Get two children to push against each other's hands. To stay in the same place, they both need to be pushing, but if one stops, he/she fall backwards. Thus the wall must be pushing against you too.

*** OTHER THINGS TO TALK ABOUT ***

Have you ever noticed your ears going pop as your train goes through a tunnel?

For the train to move forward the air in front of it has to get out of the way, normally there is lots of space for it to do this, but in a tunnel there is just a little gap around the side, which the air has to squeeze through really fast.

This fast moving air sucks some of the air out of the train, which then sucks on your eardrums, making them go pop!

Why do you think they go pop again when you leave the tunnel?

The air isn't going as fast outside so the pressure goes up and the pop back inwards again.

You may notice the ball is spinning sometimes - think about where the air must be flowing to stop the ball falling off.

This happens when the straw is at an angle so the ball drops down a little and the air stream is faster over the top which is what is holding it up.

Try moving your hands across the blower, just above the grill. This will cause the ball to 'dance' as you are changing the air stream so the beach ball adjusts its position to remain in equilibrium.

*** WIND TUNNEL ***
You can also use the blower horizontally, to demonstrate how the model plane wing flies. Children should be able to feel the air pushing the plane upwards when it is held horizontally.

One way to think about this is again with Newton's Third Law. The curvature of the wing means that the wing pushes the air blowing at it downwards, and so the air pushes the wing upwards. The size of the force depends on how fast the air is moving, and therefore how much air it is pushing down - you can show this by adjusting how hard the fan is blowing. This means that a plane has to travel VERY fast to get enough lift to keep it in the air (commercial airliners travel around 500mph).

[explanation with air pressure needs writing here]