A reflex is an action your body does without your thinking about it, like sneezing, or jumping a little when you dream you’re falling. If you suddenly catch a ball before it crashes into a window, you have quick reflexes.
When a doctor hits your knee with a little hammer during a physical, it makes your leg kick, even though you didn't consciously make your leg move. The doctor is testing your reflexes — movements that happen without your control. If your reflexes aren't working, it could mean there's something wrong with your body. Putting your hands out when you fall is a reflex. So is laughing so hard you snort.