A man went into a shop and asked for a pound of apples which cost one shilling. The shopkeeper gave them to him. Then the man asked: "Can I exchange the apples for a pound of plums? The price is the same." The shopkeeper agreed and gave him the plums. The man took them and was going to leave the shop. The shopkeeper asked him for the money.
"The money for what?" asked the man.
"The money for the plums," said the shopkeeper.
"But I gave you the apples for the plums," answered the man.
"Well, then the money for the apples."
"But you still have your apples."
After that the man walked out of the shop.