It’s not just limited to toilet paper, either. Putting the devices on bananas and coffee mugs also consistently yielded a heart rate. Those rates were also consistently faster than the 81 bpm of the toilet paper, at 77 bpm and 72 bpm, respectively. The same thing happened with other fitness trackers including the Apple Watch Series 4 and Android Wear.
So what’s going on here? As it turns out, blood absorbs green light, which is what fitness trackers use to determine heart rate. They beam a green light onto the wrist (or similarly shaped object) and use the results to calculate a pulse. The reflective surfaces of the wrist-shaped objects confused the trackers into thinking there was blood flowing through them. And the more reflective surfaces of the banana and mug explain why those would consistently yield a quicker heart rate than the toilet paper.
