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David Attenborough visits the Arizona desert where creatures like lizards are vulnerable to hawks, coyotes and cats. In the morning, when those warm-blooded animals are already active, cold-blooded animals like lizards can not move fast, so have a problem. But they also have a solution - secret sun-bathing. At David's feet, a regal horned lizard is buried up to its neck in the sand. Even while it鈥檚 buried it can use the sunshine to warm its whole body because it can control the blood supply to its head where it pools the blood in a cavity behind the eye. Soon the blood there is five degrees above the temperature of the rest of its body. At this point, the lizard opens the blood vessels in its neck, allowing the hot blood to circulate to the rest of its body which is still underground. Now it is sufficiently warmed up to run away at the first the threat of danger.