Humans Are Bioluminescent — We Emit Visible Light, Just Too Faint to See

For decades, textbooks stated humans could smell only 10,000 different odours. A landmark 2014 study published in Science overturned this, demonstrating that the human nose can distinguish at least 1 trillion distinct smell combinations. The olfactory system uses about 400 types of smell receptors, and their combination patterns allow an astronomically large number of unique smells. The sense of smell is also the only sense directly connected to the brain's limbic system, explaining why scents so powerfully trigger memories and emotions.

Gastric acid in the stomach reaches a pH of 1.5 to 3.5 — acidic enough to dissolve zinc and corrode certain metals. Yet the stomach doesn't digest itself because mucus-secreting cells continuously coat the stomach wall with a protective bicarbonate-rich mucus layer. This protection is temporary: the stomach completely replaces its inner lining every 3–4 days. If this renewal failed, gastric ulcers would quickly develop, as occurs in H. pylori infections that disrupt the mucus barrier.

Despite being called 'small', the small intestine is the longest section of the digestive tract, measuring 6–7 metres in adults. Its inner surface is covered with millions of finger-like projections called villi, and each villus is covered with even smaller microvilli (the 'brush border'), which together increase the total absorptive surface area to approximately 250–300 square metres — roughly the size of a tennis court. Food takes 2–6 hours to pass through.

The retina contains two types of photoreceptor cells: about 120 million rods (sensitive to light and dark) and 6–7 million cones (for colour and detail). The three types of cones respond to red, green, and blue wavelengths, and their combined signals allow the brain to distinguish an estimated 10 million distinct colours. The eye can also detect a single photon of light in total darkness — and can process approximately 36,000 bits of information per hour.