Rust is the reddish-brown coating that forms on iron and steel when they're exposed to air and moisture for too long. If you leave a bicycle outside in the rain or forget garden tools in the yard, that flaky orange-brown substance that appears is rust. It happens because oxygen in the air combines with the metal in a chemical reaction called oxidation, slowly eating away at the metal's surface.

Rust weakens metal over time. A rusty nail becomes brittle and can snap more easily than a clean one. A car with rust spots might eventually develop holes in its body. That's why people paint metal objects, oil their tools, or store bikes in dry places: to prevent rust from forming.

The word also works as a verb. Metal rusts when it sits exposed to weather. You might say an old swing set has rusted through, meaning rust has damaged it badly.

People also use “rust” or “rusty” to describe skills that have gotten worse from lack of practice. If you haven't played piano all summer, your playing might be rusty when school starts again. Your skills haven't disappeared; they just need some practice to work smoothly again, like cleaning rust off a tool to make it useful once more.