Contamination is the presence of something harmful or unwanted that makes something else dirty, unsafe, or impure. When a clean glass of water gets contamination from bacteria, it's no longer safe to drink. When a sterile medical instrument gets contamination from germs, doctors can't use it for surgery.

Contamination spreads from one thing to another through contact or mixing. A single drop of food coloring contaminates a whole glass of clear water. One moldy strawberry can contaminate the fresh ones touching it in the container.

Scientists worry about contamination constantly. Archaeologists wear gloves so oils from their skin don't contaminate ancient artifacts. Food manufacturers work in spotless facilities to prevent contamination of the products we eat. Environmental scientists track contamination when chemicals leak into soil or water supplies.

The word also appears in everyday situations. If you're making cookies and raw egg gets on the counter, you've created contamination that needs to be cleaned up. If someone with a cold shares your water bottle, you might say it's been contaminated with their germs. The key idea is always the same: something pure or clean has been made impure or unsafe by contact with something harmful or unwanted.