The Human Brain Has About 86 Billion Neurons With 100 Trillion Connections

Try it right now: pinch your nose and attempt to hum. It's physically impossible. Humming requires the mouth to be closed, so sound resonates through the nasal cavity and exits through the nostrils. Block the nostrils and there is nowhere for the airflow to go, preventing the vocal cords from vibrating.

Gastric acid (hydrochloric acid) has a pH of around 1.5 to 3.5 — corrosive enough to dissolve metallic zinc. The stomach doesn't digest itself because it is continuously coated by a thick mucus layer. Stomach lining cells are replaced entirely every 3–5 days to prevent acid damage.

A 2014 study published in Science (Bushdid et al., Rockefeller University) revealed that humans can discriminate at least 1 trillion distinct odors — far exceeding the long-held estimate of 10,000. The olfactory system uses around 400 different receptor types that combine to create millions of scent signatures.

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.