Okay kiddo, have you ever seen a volcano erupt on TV or in a book? Well, there are some materials that can react in a similar way, but instead of fire, they can create a big explosion! These materials are called reactive materials.
Reactive materials are usually made up of two or more things that really want to react with each other. When they get mixed together, they create a big reaction that releases a lot of energy very quickly. Kind of like how you feel when you eat a lot of candy at once and get a sugar rush!
Because reactive materials can be dangerous and cause explosions, they have to be handled very carefully. Scientists and engineers use special equipment to store and transport them safely. They also have to be very careful when mixing the different parts together.
So, in summary, reactive materials are mixtures that can create explosions and release a lot of energy quickly. They are handled carefully and with special equipment so that they don't cause any harm to people or things around them.