Temperature measures the average kinetic energy of particles in a system and is typically measured using a thermometer calibrated on scales like Celsius, Fahrenheit, or the scientific standard Kelvin. While Celsius sets water's freezing and boiling points at 0 °C and 100 °C, and Fahrenheit at 32 °F and 212 °F, the Kelvin scale is the SI base unit, with an increment equal to one degree Celsius. The theoretical lower limit is absolute zero (0 K = -273.15 °C), where classical particle motion ceases, but the third law of thermodynamics dictates it can never be reached—experiments have only gotten as low as 38 trillionths of a kelvin. Interestingly, negative absolute temperatures exist and are actually hotter than any positive temperature, defying common intuition. Temperature is fundamental across all natural sciences and daily life, influencing everything from material phase and density to chemical reaction rates, thermal radiation, and the speed of sound.