Cortisol (best known as the stress hormone) is handy for tracking your athletic performance and even spotting signs of disease, since it reflects how well your adrenal or pituitary glands are working. But there's a problem: measuring that often takes several days of lab work, by which point the info is no longer relevant. Scientists might have a much better option. They've developed a flexible, wearable sweat sensor (not shown here) that tracks cortisol levels with results in seconds -- that is, while it's at its most useful. It sounds straightforward, but the team had to overcome a major obstacle common to most biological sensors.