How does thermal expansion affect a thermometer?
Thermal expansion is the basis of which a thermometer works. This is due to the properties of thermodynamics- a substance expanding and retracting due to changes in heat.
When we think of a thermometer, one might visualize a thin glass tube with a colorful liquid in it (that lines up with a certain measurement on the thermometer). The tube is thin to allow a more precise reading. You can think of it this way: Imagine trying to suck up liquid through a thin straw and a wide straw. The liquid will go up quicker in the thin straw, because it has less space to move around in. The same is true with a thermometer. The tube is so thin that the liquid can either move up or down. And this is where thermal expansion comes in.
When a liquid is heated, the atoms will get "excited" and gain energy, making them move further apart- they have expanded. In such a thin tube, they have nowhere to go but up! Therefore, they will mark the increase in temperature for the thermometer.
The reverse is true for cooling. The atoms come closer together and take up a smaller area, so they go down in the thermometer, indicating a decrease in temperature.
On a side note...To be more specific about the rate at which a substance expands, things like specific heat and coefficients of linear expansion must be taken into consideration. The higher the coefficient of linear expansion, the more precise the reading. Why? Because that gives the highest change in length for a given change in temperature.