Tom goes to school on his bike.He put it in his bicycle stand（自行车栅）behind the school.There are many bikes there during the daytime.At half past four in the afternoon,the students take their bikes and ride home .Then the school are full of young riders on two wheels（车轮）．Sometimes the policeman stops them all and lets the cars go by.Sometimes he stops all the cars and the bicycles pass .Some students carry heavy bags of books with them．Sometimes two boys ride on one machine.The policeman always stops a bike when it is carrying two people.