The metric system is a way of measuring things using units based on the number ten. Instead of inches, feet, and miles, the metric system uses millimeters, centimeters, meters, and kilometers. Instead of ounces and pounds, it uses grams and kilograms. A meter is a bit longer than a yard, and a kilogram is about 2.2 pounds.

What makes the metric system powerful is how the units relate to each other. There are 10 millimeters in a centimeter, 100 centimeters in a meter, and 1,000 meters in a kilometer. Everything moves by tens, hundreds, or thousands, which makes calculations much simpler than converting between inches, feet, yards, and miles.

Scientists around the world use the metric system because it makes sharing measurements precise and straightforward. A chemist in Brazil and a physicist in Japan can collaborate easily because they both measure in grams and liters. Most countries adopted the metric system in the 1800s and 1900s for everyday use, too.

The United States still uses the older imperial system for most daily activities, though American scientists, doctors, and engineers use metric measurements in their work. If you've ever run a 5K race, that “K” stands for kilometers, so you're using the metric system without even thinking about it.