Chocolates. Fred’s second-favourite thing in the world, after Post-it notes.

To win the chocolates, all Gareth and Gail have to do to tell Fred how many chocolates he has in his hands. His hands will be open, but their eyes will be closed, and they have to cross their fingers for luck.

Gail first. She thinks she can feel two chocolates. Bzzzt! Wrong!

Next, Gareth. He think the same – two chocolates. Bzzt!

Fred really is only holding one chocolate in each hand. What’s going on?

Normally, when you feel something, it sits between your fingers like this. Your brain correctly works out that there must be only one object, touching the insides of two fingers.

But if you cross your fingers, the object touches the outsides of your fingers. Your brain, not used to this, gets all confused, and usually reckons there must be two objects.

This works with most round or curved objects – it can even feel weird if you cross your fingers and then stroke your nose with them. Go on, have a go.