Arthur C. Clarke once said that “any sufficiently advanced technology is indistinguishable from magic.” That’s certainly true of a mobile application that’s able to give you your heart rate simply by looking at you, with no physical contact required. No straps, nothing. Cardiio simply uses slight changes in your face’s reflectivity to calculate how fast your heart is beating. It seems the iPhone 4S’s camera is sensitive enough to detect the change in colour to the skin on your face related to an increase in blood flow, which occurs with every beat of your heart. It works with all skin tones and works best with an iPhone 4S, though other iDevices should still manage, albeit at a reduced performance.