Okay, so imagine that you have a big present in front of you, wrapped up all nice and pretty. You have no idea what's inside the box, but you're really excited to find out. Suddenly, someone says "Surprise!" and jumps out from behind a chair!
That feeling you get when someone surprises you like that is called "surprise." It's an emotion that you feel when something unexpected happens. It can be a good surprise like getting a present, or a bad surprise like a loud noise in the middle of the night.
Your brain has to work really hard when you feel surprised because it wasn't expecting whatever just happened. It has to quickly figure out what's going on and decide how you should react. Sometimes you might feel scared or happy when you're surprised, depending on what happened.
So surprise is an emotion that you feel when something unexpected happens, and your brain has to work hard to figure out what to do next.