Final answer:
As the air gets warmer, the liquid in a thermometer expands due to thermal expansion, causing the liquid to rise in the thermometer's tube and indicating a higher temperature.
Step-by-step explanation:
When the surrounding air gets warmer, the liquid in a thermometer, such as alcohol or mercury, will expand. This is due to the phenomenon of thermal expansion, which is the change in size or volume of a substance as its temperature changes.
As the liquid expands upon heating, it moves up in the narrow tube of the thermometer, which allows us to visually measure the increase in temperature. The expansion happens because molecules in the liquid move faster and spread apart as they gain kinetic energy from the heat.
Thermometers are carefully calibrated so that this expansion correlates to specific temperature readings. In solid objects like railroad tracks and bridges, thermal expansion is also accounted for with the use of expansion joints, which allow these structures to expand and contract without causing damage or deformation.