A thermometer measures temperature, which is a physical quantity that indicates the hotness of an object. There are two common types of thermometers used in school: alcohol-in-glass and mercury-in-glass thermometers. A pendulum clock works by using the consistent period of a swinging pendulum to keep time, where the period is the time taken for one oscillation and depends on the length of the pendulum but not other factors like the mass or size of the swing. Calibrating a thermometer involves marking temperature scales by comparing the instrument to standards.