We can’t say with 100% certainty, but we believe what makes Cardiio work is witchcraft. It’s either that or some technology our brains can’t quite wrap around yet. Basically, the five buck app measures your heart rate by you simply looking at your phone. By using your iPhone’s front-facing camera, Cardiio tracks the light reflect off your face and (presumably with black magic) gives you an accurate reading of your heart rate. It also lets you track your fitness level, estimate your potential life expectancy and compare your data to others. For some reason an app that costs $5 seems insanely expensive to us, but this is one that is actually worth the dough.