When we sense danger, the brain reacts instantly, sending signals that activate the nervous system. This causes physical responses, such as a faster heartbeat, rapid breathing, and an increase in blood pressure. Blood pumps to muscle groups to prepare the body for physical action (such as running or fighting). Skin sweats to keep the body cool. Some people might notice sensations in the stomach, head, chest, legs, or hands. These physical sensations of fear can be mild or strong.
This response is known as "fight or flight" because that is exactly what the body is preparing itself to do: fight off the danger or run fast to get away. The body stays in this state of fight-flight until the brain receives an "all clear" message and turns off the response.
Sometimes fear is triggered by something that is startling or unexpected (like a loud noise), even if it's not actually dangerous. That's because the fear reaction is activated instantly — a few seconds faster than the thinking part of the brain can process or evaluate what's happening. As soon as the brain gets enough information to realize there's no danger ("Oh, it's just a balloon bursting — whew!"), it turns off the fear reaction. All this can happen in seconds.