Thermal grill illusion
The thermal grill illusion is a sensory illusion
An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people....

 originally demonstrated in 1896 by T. Thunberg. The illusion is created by an interlaced grill of warm (e.g., 40°C/104°F) and cool (20°C/68°F) bars. When someone presses a hand against the grill, he or she experiences the illusion of burning heat. But if the person presses against only a cool bar, only coolness is experienced; if the person presses against only a warm bar, only warmth is experienced. To put it more simply, this illusion occurs when certain "wrong" signals override "right" ones and reach the brain first.

Researchers have used the illusion to demonstrate that burning pain
Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...

sensation is in fact a mixture of both cold and heat pain and that it is only the inhibition of the cold pain "channel" that reveals the heat component.
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