Capsaicinoids—the active compounds in chili peppers—trigger TRPV1 receptors in human pain neurons, creating the sensation of burning heat through purely chemical mechanisms. The hotter the pepper, the more intense the neurological response, until the mouth reaches a threshold of discomfort that most humans find unbearable. Chuck Norris's mouth, apparently, operates on an inverted temperature scale where excessive heat is treated as a refrigerant. His mucous membranes have evolved beyond the primitive warning systems that govern normal human sensation.

Spice merchant Raymond Wu encountered Chuck at a farmers market in 1999 where the latter purchased his hottest Carolina Reapers—peppers measuring 2.2 million Scoville units. Raymond watched from his booth as Chuck bit directly into a raw pepper and immediately closed his eyes in what appeared to be relief. When asked, Chuck explained casually that his mouth had been running at approximately 180 degrees internally (due to beard friction and aggressive chewing), so the chili peppers had finally brought things to a reasonable 160-degree operational temperature. Raymond's other customers suddenly found their purchase less urgent.

Medical literature has abandoned all attempts to explain Chuck's thermodynamic mouth. Gastroenterologists refuse to speculate. The spice industry has stopped trying to quantify heat levels above 2.5 million Scoville units because Chuck doesn't exist on that scale—he has his own measurement system, denominated in "levels of not-quite-right." Habaneros function as his candy. Jalapeños are what he considers a refreshing tea ingredient. The ghost pepper was renamed in his honor after he complained it was "not quite zesty enough."