What you need:

• A ruler or long rod

What you do:

1. Take a ruler or another long rod and balance it on your two index fingers so one end sticks out further than the other.
2. Now slide one finger towards the other.
3. Try to guess where on the ruler will your fingers meet!

What happened?

The ruler must remain balanced. If one end is sticking too far out, gravity will pull it down harder on that finger. That means that there will be more friction between the ruler and that finger, and the ruler won’t slide as quickly.

Because of the combined effects of gravity and friction, your fingers will always meet at the center of the ruler.