What if your morning run through a park could generate electricity, which could then charge batteries or maybe even power the phone you’re running with? If you’re wearing the right T-shirt, that might be a possibility.

Researchers from the University of Málaga in Spain and the Italian Institute of Technology have designed a T-shirt that generates electricity from the temperature difference between the wearer’s body and their surroundings.

That means the body heat coming off you when you run, walk, or play a sport—and the difference between that heat and the colder temperature of the surrounding air—could be transformed into thermal power.

There are a lot of examples of thermal energy in our lives—the sun warming our atmosphere, a stove burner heating a pot of water to boil.

But there are also a lot of examples of wasted warmth, or “waste heat,” which is all the unused heat let out into an environment by machines, electrical processes, or even human activity, that could be used as thermal energy.

You can make use of this heat, though, by using something called the thermoelectric effect, which lets you convert temperature differences into electric voltage.

If two conductors are connected and one side is heated, electrons start to move over to the cooler side, creating a current that flows through the circuit.

Some materials act as conductors that can convert that temperature difference into a power source.

What this T-shirt does, researchers say, is capture heat coming off of our bodies, which contrasts with the cool air, making electricity.