 Can you tell whether these cells are healthy or will soon die? For years, scientists performing fluorescence microscopy experiments assumed cells like these were healthy, but researchers from Germany report that many of these seemingly healthy cells are in the process of dying. Fluorescence microscopy uses lasers to excite photoreactive molecules, which then emit light to create a picture. In the past, cells appearing healthy right after laser exposure were deemed suitable for study. However, many of these cells have actually suffered massive damage and will die the following day. Using well-known cell lines, the group tested commonly used laser settings, but instead of monitoring cell health for the usual one to two hours after laser exposure, they watched cells for a whole day. They found that many cells initially appearing healthy had suffered light-induced damage that killed them within 24 hours. This finding challenges past beliefs about microscopy. It's been accepted that exposure to intense light can destroy photoreactive molecules, but the idea that light causes delayed damage to cells hasn't been widely considered. These damaged cells likely act differently than healthy cells, and studying them can lead to incorrect conclusions. But the researchers reported good news, too. They were able to pinpoint laser settings that don't seem to cause much damage. They also found a way to see the first signs of damage that didn't require a day of monitoring. By looking at the behavior of small protein tubes called microtubules, which help cells move, they could tell immediately after light exposure whether a cell was likely to die. These findings show that timing is everything in science. Knowing when to look can mean the difference between studying any irreparably damaged cell and one that can give meaningful results. But they also show that by making small changes, such as shifting the wavelength or timing of light, scientists can tweak their experiments to keep cells healthy.