Milli is a prefix that means one-thousandth of something. When you see milli at the start of a word, it tells you to divide the base unit by 1,000.

A millimeter is one-thousandth of a meter: 1,000 millimeters line up to make one meter. A milliliter is one-thousandth of a liter: you'd need 1,000 milliliters to fill a one-liter bottle. A millisecond is one-thousandth of a second, so brief that 1,000 of them flash by in a single second.

Scientists use milli because it's more practical than writing out tiny decimals. Instead of saying something is 0.005 meters long, they say it's 5 millimeters. Medicine doses often use milligrams because pills contain such small amounts of active ingredients.

When you're measuring something small, like the thickness of a coin, the width of a fingernail, or the amount of medicine in a tablet, milli units make the numbers manageable and easy to understand.