The boiling point of water is constant given a constant physical environment, but it varies from place to place and even in the same place. The boiling point of any liquid depends on the atmospheric pressure bearing down on its surface: the higher the pressure, the more energy it takes for liquid molecules to escape the surface and become a gas, and so the higher the temperature at which the liquid boils. Every 1,000 feet/305 meters in elevation above sea level lowers the boiling point about 2°F below the standard 212°F (or 1°C below 100°C). And food takes longer to cook at 200° than it does at 212°. Even a low-pressure weather front can lower the boiling point, or a high-pressure front raise it, by as much as a degree or two.