The sound of a cracking knee isn’t particularly pleasant. But it gets worse when you listen up close. “It does for most people. But for me, it just makes me excited.” Omer Inan, an electrical engineer at Georgia Tech. “I actually feel like there’s some real information in them that can be exploited for the purposes of helping people with rehab.” Inan’s experience with cracking knees goes back to his days as an undergrad at Stanford, where he threw discus（铁饼）. “If I had a really hard workout, then the next day of course I’d be sore, but I’d also sometimes feel this basically catching or popping or creaking every now and then in my knee.” A few years later, he found himself building tiny microphones at a high-end audio company. So when he got to Georgia Tech and heard the Defense Advanced Research Projects Agency, DARPA, wanted better tech for knee injuries, he thought: Why not strap tiny microphones to people’s knees, to eavesdrop（偷听，窃听） as their legs bend?” What we think it is, is the cartilage（软骨） and bone rubbing against each other, the surfaces inside the knee rubbing against each other, during those movements.” He and a team of physiologists and engineers built a prototype with stretchy athletic tape and a few tiny mics and skin sensors. And preliminary tests on athletes suggest the squishy sounds the device picks up are more erratic（不稳定的）, and more irregular, in an injured knee than in a healthy one, which Inan says might allow patients and doctors to track healing after surgery. Details appear in the IEEE Transactions on Biomedical Engineering. “The primary application we’re targeting at first is to give people a decision aid during rehabilitation, following an acute knee injury, to help them understand when they can perform particular activities, and when they can move to different intensities of particular activities.” A useful thing to take a crack at.