To contaminate something means to make it dirty, impure, or unsafe by adding something harmful or unwanted. When bacteria contaminate food, the food becomes unsafe to eat. When chemicals contaminate drinking water, the water is no longer clean. When someone with a cold touches a doorknob without washing their hands, they might contaminate it with germs.

Scientists and doctors work hard to prevent contamination. In a laboratory, researchers wear gloves and use sterile equipment to keep their experiments from being contaminated by outside substances. Surgeons scrub their hands thoroughly and wear masks to avoid contaminating the operating area with bacteria.

Contamination usually happens through contact or mixing. A single drop of oil can contaminate a whole glass of water, making it undrinkable.

You'll hear about contaminated soil at old factory sites, contaminated evidence at crime scenes (where someone accidentally touched something they shouldn't have), or contaminated samples in science class. Once something is contaminated, you usually can't use it anymore. That's why preventing contamination matters so much in hospitals, kitchens, and science labs.