Celsius is a temperature scale used by most countries in the world to measure how hot or cold something is. On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees. These two reference points make it easy to understand: if the temperature outside drops below 0°C, you might see ice forming on puddles. If it climbs to 30°C, that's a hot summer day.

The scale was invented by Swedish astronomer Anders Celsius in 1742. He wanted a simple, logical system based on the behavior of water, since water is something everyone experiences daily. Before Celsius, different countries used different temperature scales, which made scientific communication confusing.

Most of the world uses Celsius for everyday weather forecasts, cooking, and science. If you're checking the weather in Paris, Tokyo, or Sydney, you'll see temperatures in Celsius. The United States mostly uses a different scale called Fahrenheit, where water freezes at 32°F and boils at 212°F. Scientists everywhere use Celsius (or its close relative, Kelvin) because it makes calculations and comparisons easier.

When you hear that normal human body temperature is about 37°C, or that a comfortable room is around 20°C, those numbers come from this scale. Understanding Celsius helps you know what to expect from the weather and communicate about temperature with most of the world.