Oswald Mega, the always welcome, has two envelopes and some money. He divides the money into three equal stacks.
He puts one of the stacks in one envelope, and the rest of the money into the other. He seals the envelopes, takes them to the usual watering hole, describes what he has done, swaps them around behind his back a few times, and then he hands you one of them.
The envelope contains £1000.
"Would you like to swap envelopes?" he says.