Researchers have no settled answer. Leading ideas link yawning to brain arousal and cooling, and contagious yawning to empathy, group synchrony and vigilance.
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Brain freeze is a brief headache set off by very cold food or drink on the palate. Its exact mechanism is still debated, involving the trigeminal nerve and blood flow.
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Historian Roger Ekirch argues segmented sleep was the predominant pre-industrial Western pattern. Historian Niall Boyce has challenged that, and a study of three pre-industrial societies found no such pattern.
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Stuck songs, or earworms, arise when a familiar, repetitive tune is cued by a memory, a mood or recent listening and then replays on a mental loop. Researchers still lack a full theory.
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A hiccup is an involuntary diaphragm spasm followed by a sudden closing of the vocal cords. The triggers are well known, but why the reflex exists is still unsettled.
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Wrinkling is mainly an active nerve-driven response: blood vessels in the fingertips narrow and the skin puckers. It may help grip wet objects, though that purpose is still debated.
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Goosebumps are an involuntary reflex in which tiny muscles pull hairs upright. It is a leftover from furrier ancestors, triggered by cold and by strong emotion.
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Blushing is a burst of blood flow to the skin, triggered by the sympathetic nervous system during emotions like embarrassment. Why it evolved is still debated.
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Researchers think emotional tears mainly work as a social signal that draws support and bonding, but the underlying biology is not fully understood.
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No single cause of déjà vu is settled. Leading explanations describe a memory-familiarity misfire, checked by the frontal cortex, or a resemblance to a forgotten scene.
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No single explanation is settled. Memory consolidation is the leading theory, but emotion processing, threat rehearsal and 'no purpose at all' remain live alternatives.
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Cutting an onion triggers enzymes that make an airborne sulfur irritant, propanethial-S-oxide. It stings the eye's surface nerves, and the tears wash it away.
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A foot falls asleep when pressure on a nerve disrupts its signals. The tingling is the nerve recovering once pressure lifts. Sources differ on how much blood flow matters.
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Handedness is shaped partly by many genes, each with a small effect, and partly by chance variation in early development. No single cause or left-handedness gene exists.
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