Footballs
A man walks into a sports shop to buy a football. He goes straight to the counter, puts down $10, and asks for one of the special offer footballs. The sales assistant asks him which ball he would like, and then gets it from the other room. The customer takes his premium ball and leaves. About 15 minutes later, another customer goes into the shop for a special offer football. He puts his $10 on the counter and asks for a football. He doesn't say what kind he would like and the assistant doesn't ask him. He just gives the customer the premium ball and takes his money. This is the ball he wanted, but how did the assistant know?
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There was a special offer price list on the sports shop window. The basic ball was $6, the deluxe ball was $8, and the premium ball was $10. The first customer gave the assistant a $10 note, so he could have wanted any of those balls. The second handed over a $5 note and five $1 coins, so he obviously wanted a premium ball of $10 (having only stated that he wanted a ball).