Researchers at Columbia University have figured out how to make machines powered by evaporating water. How does that work? Well, as it turns out, certain bacterial spores grow and shrink based on how much water they have access to. In more human environments they grow, and in drier environments they shrink.

What the scientists did was take some thin tape, like you might find in a cassette, and apply spores to it in a series of dashed lines on either side, so that lines on one side filled the gaps between the lines on the other side. When the spores could take in water, they expanded and the tape became slack, but when the water evaporated, they would contract and tighten the tape, pulling on anything the tape was attached to.

For one machine, they put a series of these tapes into a small plastic case with shutters. When the case grew humid the tape slackened, and the shutters would open, allowing the water to evaporate. As the water evaporated the tapes would tighten, closing the shutters and allowing the case to become humid again. Using this method, they managed to power a small light using just the action of the tapes contracting and expand.

They also built a small engine using the same principles, which was able to power a toy car. The “moisture mill” which powered this car had tapes that were half inside and half outside of a case, with the inside exposed to water. This caused the tapes to constantly contract and expand, letting the mill keep turning to power the car.

The researchers suggest that this concept could be made at a much larger scale, allowing for generators that sit above water to create electricity or for cars that don’t require fuel or an electrical battery. They suggested that generators of this nature could produce as well as wind turbines.