Q: Humans are the only known species to do this and it can’t be faked. However, we spend $70 million a year to mimic it. What is it?

A: Blush. The sympathetic nervous system triggers blood vessel dilation in the face (and other areas, including internal tissues like the stomach lining), causing the visible redness associated with blushing. It’s an involuntary response, which is why it’s difficult to fake convincingly. In the middle ages women used water and strawberries as blush.