Stress
Stress is how your body reacts when something pushes you to respond fast.
Think of it this way
A pot of water sits on a hot burner. The water starts to bubble and move fast to release the heat. Your body does the same thing when pressure hits — it speeds up and reacts. When the burner turns off, the water calms down again.
Explaining stress by grade level
Your body can feel stress. When you are cold, your hands get slow. Try to catch something fast. Stress makes it harder to move quick.