How do you know when exercise is burning fat? You don’t, actually, but soon you may. Researchers at ETH Zurich, Switzerland’s federal institute of technology, have developed a breath sensor that measures biomarkers (mainly acetone) in our exhalations, a reliable indicator of the amount of fat being burned. Futurity reports on the research, which makes it possible for people to get immediate readouts of the rate at which their bodies are burning fat. No you can’t buy it yet, but the researchers do hope to bring a version of the device to market.