Understand the world, one question at a time.

Cutting an onion breaks its cells, letting sulphur compounds react with a series of enzymes. The end product is a volatile gas called syn-propanethial-S-oxide that drifts into the air. It stimulates nerve endings in your eyes, and your brain responds by producing tears to protect them.

Nobody knows the purpose of dreams for certain. Leading theories suggest dreams help the brain sort and store the day’s experiences, process emotions and rehearse responses to possible threats. The most vivid dreams happen during REM sleep, but dreaming also occurs in other sleep stages.

Rainwater slowly wears down rocks and dissolves their minerals, and rivers carry these dissolved salts to the sea. When the Sun evaporates seawater, the water leaves but the salt stays behind. Hot springs on the ocean floor and undersea volcanoes add more. Built up over millions of years, these salts make up about 3.5% of seawater.

Cats purr by vibrating their vocal folds roughly 20–30 times per second. A 2023 study showed that special soft tissue in the vocal folds can produce this low frequency on its own. Purring often signals contentment, but cats also purr when frightened, in pain or asking for something.