UFO watchers' eyes were set ablaze recently by reports of what looked like a stony rodent lurking among the rocks on Mars. The so-called Mars rat, spotted in an image taken last year by the NASA rover Curiosity’s Mast Camera, captured imaginations even as it inspired several new parody Twitter accounts.

But, just so it's clear, this Red Planet rodent – which looks rather more like a guinea pig to this reporter’s eye – is no more real than the Man in the Moon. In fact, the rat likely comes from the same source as that lunar visage: the human brain.

Humans experience a phenomenon known as pareidolia, which is the tendency to distinguish animals, faces or other significant images in otherwise inanimate objects. This ability to pick out faces quickly is likely a product of evolution, as Carl Sagan explained in "Demon-Haunted World: Science as a Candle in the Dark."

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