教科 p.54-55
About 700 years ago, people started to make mechanical clocks. At first, they used weights to
power these clocks. The clocks told the hour with bells. However, the weights were big and
heavy, so (were/to/difficult/clocks/move / these ).
About 500 years ago, people improved their clocks. They used springs to power the clocks. The
springs were small and light. So people could move the clocks easily. Eventually, people
to carry watches.
began
Today, we have clocks and watches everywhere. This is the result of many great inventions and
