One out of every ten newborn infants displays a weird quirk of the human body called "harlequin sign." It makes babies spontaneously turn parti-coloured, especially when their tiny bodies heat up.
Harlequin sign begins freaking parents out at two days after the child's birth. When the parents, for a moment, lay their baby on his or her side, half of their kid turns bright red. And when we say half, we mean exactly half. One of the features of harlequin sign is it neatly bisects the body along a straight line.
Meanwhile, the side of the body that doesn't go red is extremely pale, as if a new baby were intentionally trying to make the colour difference as terrifying as possible for its nervous parents. The colour change dissipates fairly quickly. If parents are lucky it goes away after half a minute, but it can last as long twenty minutes, at which point, many panicked parents will be clawing at the doors of the emergency room.
Newborn blood vessels are highly sensitive to even slight changes in temperature or body position (or even mood), constricting or expanding with a corresponding change in blood flow. In newborns prone to Harlequin sign, this manifests as a colour change in the skin.
Occasionally this syndrome does occur with complications, but most babies with harlequin sign — including the baby girl pictured above — have a bad case of the absolutely fines. Eventually the sign goes away on its own.
At least that's the case with newborns. Older children, and stroke victims, sometimes develop harlequin sign as a result of Horner's syndrome — the disruption of the nerve pathways to one side of the face. Because this disruption isn't restricted to the skin, people with this sign also have one eyelid that droops a little, and one pupil that doesn't respond to light the way the other one does. Although it can be embarrassing, the syndrome isn't painful or debilitating, and clears up when the nerve disruption itself is treated or heals.