Reactivity is how readily something responds to or interacts with other substances or conditions. In chemistry, reactivity describes how quickly or easily a substance undergoes chemical reactions. Sodium has high reactivity: drop it in water and it fizzes violently, releasing hydrogen gas. Gold has low reactivity: that's why ancient gold coins can sit buried for thousands of years and still gleam when discovered.

The concept appears in other fields too. In psychology, emotional reactivity describes how quickly or intensely someone responds to situations. A person with high emotional reactivity might burst into tears at small disappointments or flare up in anger over minor frustrations. Someone with lower reactivity stays calmer when things go wrong.

Scientists must account for reactivity when designing experiments. If you're studying how students behave during recess, they might act differently simply because they know they're being watched. This is called reactivity to observation: people change their behavior when they know someone's paying attention.

Understanding reactivity helps predict what will happen when things interact. Chemists store highly reactive materials carefully because they know those substances might react dangerously with air, water, or other chemicals. Teachers recognize that some students are more reactive to criticism or praise than others, and adjust their approach accordingly.