Why do we see faces in clouds, toast and even Mars?

Because your brain runs a face detector with a hair trigger. Pareidolia is the tendency to perceive something meaningful, like a face, an animal or an object, in a vague pattern where there is nothing of the sort. Clouds, the Man in the Moon, a smiling power socket: the brain happily supplies eyes and a mouth. It isn't only a human quirk, either. Rhesus macaques see faces in things too.

It happens fast and automatically, not because you decide to play a game. In a 2009 brain study, face-like objects switched on the brain's face area, the fusiform face area, about 165 milliseconds after they appeared, close to the 130 milliseconds it takes for a real face. Ordinary objects didn't trigger it at all. One leading idea for why: our ancestors who read faces quickly, especially threatening ones, had a head start to flee or strike first.

The most famous false alarm happened on Mars. In 1976 the Viking 1 orbiter photographed a two-kilometre mesa that looked just like a human face. Viking's chief scientist called it a "trick of light and shadow", and sharper images later showed a natural hill whose face depends on the viewing angle and the lighting.

What we expect shapes what we find. In one experiment, people looking at plain linen who were told it had religious importance, and given hints, reported 37 religious words in it. People merely told there might be words saw 2.