A reaction is a response to something that happens. When your friend tells a funny joke and you laugh, that's a reaction. When you touch a hot stove and instantly jerk your hand away, that's a reaction too. Your body and mind are constantly reacting to the world around you.

Reactions can be physical, like when your pupils get smaller in bright light, or emotional, like feeling nervous before giving a speech. They can be quick and automatic (flinching when something flies at your face) or slower and more thoughtful (deciding how to respond when someone criticizes your work).

In science, a chemical reaction happens when substances combine or change to form something new. When you mix vinegar and baking soda and they fizz up, that's a chemical reaction. When iron rusts or wood burns, chemical reactions are taking place.

The word also appears in phrases like chain reaction, where one event triggers another, which triggers another, like dominoes falling in sequence. People sometimes talk about getting a reaction out of someone, meaning they're trying to provoke a response. If a movie gets strong reactions from audiences, it means people have powerful feelings about it, whether positive or negative.

Notice that reactions can reveal something true: they may show what you think or feel in the moment, before you have time to hide it or think too carefully about it.