The volume of cells can vary dramatically. Similarly to an inflating balloon, the volume increase of growing cells pushes on the plasma membrane—the lipid envelope that surrounds the cell. This “turgor” pressure increases the tension of the membrane, which, if left uncorrected, will ultimately cause the cell to burst. To prevent this from happening, cells have evolved mechanisms to monitor the tension of their plasma membrane. When tension is too high, cells respond by increasing the amount of lipid in the membrane. Conversely, when tension is too low, cells remove lipid from the membrane to “tighten” it. How cells manage to sense tension and trigger the appropriate biological response has remained a mystery. It has been difficult to solve due to a lack of tools to study membrane tension within living cells. To tackle this problem, researchers from the University of Geneva (UNIGE) and the National Centre for Competence in Research Chemical Biology (NCCR) have collaborated to create a fluorescent molecule to measure the tension of the plasma membrane of live cells. Using this new tool, they were then able to discover how cells adapt their surface to their volume. These results, published in Nature Chemistry and Nature Cell Biology, pave the way for many applications, including in the detection of cancer cells that typically display aberrantly high membrane tension.