One day, a little schoolboy went home with a pain in his stomach.

    "Well, sit down and 1 a cake," said his mother."Your stomach 2hurting because it's empty.It'll be all right when you've got something in it."

    Five minutes later, Dad came home 3 the office.And he said he had a headache.

    "That's because your head is 4," said his clever son."You'll be all right if you have got 5 in it." His dad and mum laughed.

证明：通过水管放水，当流速相同时，如果水管截面(指横截面，下同)的周长相等，那么在相同时间里截面是圆的水管比截面是正方形的水管流量大.