Athermometer that uses a liquid to measure temperature works because

The liquid in a thermometer is sometimes colored alcohol but can also be a metallic liquid called mercury. Mercury and alcohol expand when heated and shrink when cooled. Inside the glass tube of a thermometer, the liquid has no place to go but up when the temperature is hot and down when the temperature is cold. That's how a thermometer that uses liquid to measure temperature.

A rule of thumb is that liquid expands when heated, so when liquid is in a given space one can measure how far the liquid moves when it gets heated.