What property of matter best describes the way a typical alcohol thermometer works?

Explanation:

Matter expands when heated.

Matter is composed of particles (atoms, molecules or ions) which ocuppy a space.

At lower temperatures, matter has low kinetic energy, so the particles vibrate and or translate with low speed. When the matter is heated, the kinetic energy of the particles increases, meaning that they will vibrate and move faster.

When the particles vibrate and move faster they will occupy more space, which explains the thermal expansion.

So, at start the alcohol thermometer will be in thermal equilibrium with the surroundings (same temperature). The alcohol will occupy a certain volume inside the thermometer, which corresponds to a level mark (altitude of the alcohol inside the thermometer)

If the thermometer gets in contact with a colder substance, the walls of the thermometer will transfer heat to the colder substance, and the alcohol inside the thermometer will transfer heat to the walls of the thermometer. As result, the alcohol will lose kinetic energy and it will occupy less volume, which will cause that its level inside the thermometer fall.

On the other hand, if the thermometer gets in contact with a warmer substance, the heat will flow from the warmer substance to the alcohol inside the thermometer, the particles of alcohol will vibrate and move faster, its volume will increase and, being as it is contained inside the walls of the thermometer, its level will rise.