When you do some running, you have a lot of energy which comes from the movement of your body. When you suddenly stop, your body loses this energy. We already have watches and small medical devises (设备) which can use energy that we make when we move. In the future, people like police officers and soldiers may wear devices on their legs to "catch" this lost energy and keep it in batteries (电池). They could use the power for computers, radios or other devices.

In December 2008, most people walking across Hachiko Square, Tokyo, probably did not notice four yellow squares on the road as they hurried to work. The squares were made of special materials that make electricity when they change shape. When people stood on the squares, the shape of the materials changed and they produced electricity. The squares were only there for twenty days, but in that time they produced enough power to make a TV work 1,400 hours! Imagine putting these squares under all the streets in Tokyo. One day, we may turn our streets into power stations!

We can also use body heat. Every day, 250,000 people use Stockholm's Central Station. They eat and drink, carry heavy bags, and run to catch trains—and their bodies produce a lot of heat when they do these things. Inside the station, the heat is taken from the air and used to heat water for a nearby building.

It is a great way to get free energy—all you need is a lot of people.