What you Need
What to do
Put the tennis ball in one plastic bag and tie up the loose bag with a couple of knots to take up the excess plastic so it doesn't act as a parachute.
Make a tube of some kind which the string can move though easily. You could use a pen tube or the top of a squash bottle.
Feed the string through the tube and tie it to the other plastic bag. so the whole arrangement is a little longer than you are tall.
Put a weight in the second bag.
Now hold the tube and use it to swing the ball around your head.
What happens to the weight?
Try adding more or less weight to the second bag and see how it affects the ball.
What may happen
You should find that the tennis ball can lift up the much heavier weight. And the more weight you add the faster the ball has to go, and the closer the ball is to the tube the faster it has to rotate to support the weight.
Why does it happen?
Issac newton worked out that all objects will continue in a straight line unless you apply a force to them and this is the case with the tennis ball, the only way that you can make it go around a corner is to apply a force to it sideways.
When you are whirling the tennis ball around your head the string will force the ball to go in a circle, this will stretch the string putting it under tension. This tension will then lift the weight.
The faster the tennis ball is moving the more force you need to apply to it to go around the same corner, so if you try and speed up the tennis ball this needs more force than the weight can provide and the tennis ball will lift the weight. This means that the tennis ball is moving in a larger circle and therefore it needs less force to turn a less sharp corner and it reaches equilibrium again. This also means that if you add more weight the ball has to be going faster to lift it.
So what is centrifugal force?
If something is being held on a rotating object and then it's released it will now be able to continue to move in a straight line. For example, imagine you were standing on a roundabout at a playground, and holding a ball. If you let go of the ball it will move away from you.
But from your perspective, standing on the rotating object, the ball you release would appear to accelerate outwards as if it was being pushed by a force. Although the object it is actually moving in a straight line you are moving along a curve so you accelerate away from it, giving the appearance that it is accelerating away from you.
Sometimes it is easier to think of a world with centrifugal and coreolis forces (another pseudo force from rotations, which drives winds and waves in the opposite direction to the turn of the Earth) rather than everything moving. They are called 'pseudo forces' because they are a consequence of how you are looking at the problem.