William Thomson, Lord Kelvin (1824–1907) was a Scots-Irish physicist and engineering whirlwind who published his first scientific paper at sixteen and never really stopped. He helped lay the first working telegraph cable across the Atlantic, held dozens of patents, and thought so deeply about heat and temperature that a whole temperature scale is named after him.

The coldest cold there is

Kelvin worked out that there's a genuine bottom to temperature — absolute zero — the point where particles have as little energy as nature allows, at about -273.15\,^{\circ}\text{C}. Start counting from there and you get the kelvin scale, the one scientists use whenever they reason about the gas laws, where pressure and volume behave beautifully simply only if temperature is measured from absolute zero rather than from freezing water.