There are actually many kinds of thermometers available, but let’s talk about the two types that you probably have in your home.

The first type is a glass tube filled with either alcohol or mercury. As the temperature rises the volume of the liquid also increases, so it rises up the tube. The height of the liquid is calibrated with a scale so you can read the temperature value easily.

The second type is known as a bimetallic strip thermometer. While you’ve probably never heard this name before, you’ve likely used this type of thermometer. They are the most common models of thermostats (before they went digital), used as meat thermometers, oven thermometers, and the little thermometers that you see baristas using at coffee shops when they’re steaming milk for your latte. You can tell from the name bimetallic that there are two metals involved here (usually steel and copper). In order to measure temperatures, these two metals need to expand at different rates when they are heated. If you make a strip of these two metals and wind that strip up into a coil, the difference in their thermal expansion will cause the coil to wind tighter or unwind as the temperature changes (depending on which side of the coil you place the material that expands more). This coil then turns a needle to indicate that the temperature is rising or falling.