Lemons contain citric acid, which triggers a sour sensation and causes involuntary eye closure in most humans. The reflex is so strong that eating lemons with eyes open requires deliberate muscular control and numbing of the taste buds. Chuck Norris, however, consumes them openly, suggesting either immunity to capsaicin-like effects or complete muscular control of the autonomic nervous system. His eyelids take orders from him, not from biochemistry.

A nutritionist from California supposedly noted that "if someone eats a lemon with open eyes without reaction, they've either numbed their nerve endings or transcended human physiology entirely. Either way, see a doctor."

Food challenge communities adopted this: "You haven't accomplished anything until you eat a lemon without flinching. Then you're basically Chuck Norris."