A square root is the number that, when multiplied by itself, gives you a particular number. The square root of 16 is 4, because 4 × 4 = 16. The square root of 25 is 5, because 5 × 5 = 25.

Think of it as working backwards from squaring a number. When you square 6, you get 36. When you find the square root of 36, you discover it's 6. The symbol for square root looks like a checkmark with a line across the top: √.

Most numbers have square roots that are messy decimals that go on forever. The square root of 2 is about 1.414, and the square root of 10 is about 3.162. But certain numbers, called perfect squares, have square roots that are clean whole numbers: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, and so on.

Square roots appear throughout mathematics and science. Architects use them to calculate diagonal distances. Physicists use them in formulas about energy and motion. Even video game programmers use square roots to calculate distances between characters on screen. The ancient Babylonians figured out ways to calculate square roots nearly 4,000 years ago, long before calculators existed.