Okay, kiddo, you know how some stories have things in them that aren't really real, like talking animals or magic spells? Well, sometimes in those made-up stories, people include a substance called mercury.
Mercury is a type of metal that is often silver and liquid at room temperature, which means it easily moves around like water. It can be dangerous to people if they touch it or breathe it in, so we have to be careful with it in real life.
But in fiction, people use mercury in all sorts of ways. Sometimes it's a magical liquid that can grant special powers or make people immortal. Other times, it's a deadly poison that villains might use to hurt someone.
Mercury is also sometimes used in real life products, like thermometers, which help us measure our body temperature when we're sick. But we still have to be careful with it, just like in the stories.