Explain, at the particle level, how a thermometer measures the temperature of the system.
Matter expands and contracts with changes in temperature because a change in thermal energy causes the molecules to speed up (or slow down).  When in contact with warmer objects, energy from the surroundings is transferred to the glass of the thermometer.  The energy travels through the glass, as each glass particle begins to move faster and farther apart. Finally, the heat energy reaches the alcohol molecules in the bulb of the thermometer; and as a result, the alcohol particles begin to move faster and farther apart. This causes the alcohol to expand and rise in the capillary. When the bulb is placed in contact with cooler objects, energy is transferred from the alcohol to the surroundings; the molecules move slower and closer together. The resulting contraction causes the level to fall in the capillary.