Turning sweat into watts

Walk into a spinning class at the New York sports clubs facility on Eighth Avenue and West 23rd street in Manhattan and you´ll find 20 sweaty people furiously pedaling their stationary bikes. Look closely and you´ll notice something unusual about this workout: Each of the bikes is attached to a black box with wires running out of it. The box is a compact generator that converts the motion of the wheels into electricity, which is then fed into the power grid, offsetting some of the club´s energy use. For these gym-goers, it´s not just about their cardio fitness; their sweat is helping to make the planet a bit greener.