You probably see wheels underneath cars, airplanes, bikes and many other vehicles. 你可能看到过汽车、飞机、自行车和很多其他交通工具底下的轮子。

　　A wheel is a kind of machine. 轮子是一种机器。

　　It is round, it moves along the ground. 它是圆形的，能沿着地面滚动。

　　It helps people to move heavy things, also it helps people to travel faster. 它能帮助人们运送笨重的东西，也能使人们的旅行更快捷。

　　Do you know when the first wheel was made? 你知道第一个轮子是什么时候制造出来的吗？

　　It was made around B.C.3500. 它大约是在公元前3500年制造的。

　　At that time, it was made of wood. 那时，它是由木头制成的。