星期天,午睡后明明和爸爸上街买菜。马路上,人群熙熙攘攘,自行车、摩托车、大卡车、小卧车来来往往如同穿梭。忽然,明明向爸爸提出一个问题:“爸爸,您说为什么一般的汽车、自行车、摩托车或者三轮车的前轮和后轮一样大,偏偏拖拉机的轮子是前面的小、后面的大呢?” On sunday, it is obvious that my father and father took to the streets to buy food after going to bed. On the road, the crowd bustling, bicycles, motorcycles, trucks, small cars come and go like shuttle. Suddenly, the father clearly asked a question: “Dad, why do you say the general car, bicycle, motorcycle or tricycle front and rear wheels as large, but why the tractor wheels in front of the small, behind the big? ”