Mr. Green wants to buy a cap. He goes to a shop for a cap. He does not know the size(尺寸). The salesman shows a cap to him. I But it is too large. The size was eight. It comes down over his ears and eyes. The second cap is too small to him. The third one is the right size, but Mr. Green doesn't like the colour. So the shop assistant goes to the back of the shop and comes back with a black cap. The size is seven and a half. That is the right colour and right size. So Mr. Green buys it.