An art teacher asks his students to draw pictures. They can draw anything (无论什么东西) they want.

    After a while, a boy hands in(上交) his picture. The teacher looks at it but sees nothing(没有什么东西) on the paper.

    "What do you draw?" the teacher asks.

    "A cow eats grass." the boy answers.

    "But where is the grass?" the teacher asks again.

    "The cow has eaten it up." answers the boy.

    "Where is the cow?"

    "When it eats up the grass, it feels thirsty and goes to drink water."