Materials that light up or change color in response to stress could enable responsive clothing or yield status reports on the condition of electronic skin. But many force-sensitive materials produce colors by breaking chemical bonds, making the changes hard to reverse and limiting the materials’ reusability. Now, an international team of researchers has made polyurethanes that glow a variety of colors when stretched and then instantly switch off when relaxed, thanks to mechanically interlocked molecular structures called rotaxanes (ACS Cent. Sci. 2019, DOI: 10.1021/acscentsci.9b00173). The team made blue-, green-, and orange-glowing polymers and combined them to produce a white one.