Celsius sets freezing at zero and boiling at one hundred for pure water at sea level.

The longer version

The Celsius scale takes two everyday reference points and divides the gap between them into a hundred equal steps. Pure water at normal atmospheric pressure provides both anchors: it freezes at the lower mark and boils at the upper one. The spacing is linear, so each step represents the same change in energy.

Kelvin uses steps of the same size but starts at the coldest temperature possible, so water freezes at a value roughly two hundred and seventy-three above its zero. Scientific work favours that scale because it avoids negative numbers. Weather reports still favour the older scale in most countries.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.