Why this happens in the first place, however, is not so cut-and-dry. You might assume those yellow marks are courtesy of the sweat and oil that accumulate after wearing the same T-shirt four days in a row. (Oh come on, don't act like you've never done that.) And while that does play a minor role, the research and development team over at Degree tells us there's way more to it than that. Even if your sole exercise consists of thrice-daily walks from your bedroom to your bathroom, you're still not safe — because it's actually from the mixture of chemicals in your laundry detergent and your anti-perspirant deodorant.