Freezing point is the temperature at which a liquid turns into a solid. For water, this happens at 32 degrees Fahrenheit (or 0 degrees Celsius). When you put a glass of water in the freezer, it stays liquid until it cools down to 32°F, and then it starts turning into ice.

Different liquids have different freezing points. Mercury, the silvery liquid inside old-fashioned thermometers, doesn't freeze until it gets incredibly cold: about minus 38°F. That's one reason thermometers used mercury instead of water. Olive oil can start to solidify around refrigerator temperatures, which is why it sometimes gets cloudy and thick in your kitchen cabinet during winter.

The opposite of freezing point is melting point, which is the temperature where a solid turns back into a liquid. For any pure substance, these two temperatures are exactly the same. Ice melts at 32°F, and water freezes at 32°F.

Scientists care about freezing points because they help identify unknown substances and understand how materials behave. If you found a mysterious liquid and measured that it froze at exactly 32°F, you'd have good evidence it was pure water. Adding salt or other substances to water lowers its freezing point, which is why people spread salt on icy roads in winter: the salt makes it harder for water to freeze, helping melt the ice even when the temperature stays below 32°F.