Caffeine
Caffeine is a substance in coffee and tea that makes your brain feel more awake and alert.
Think of it this way
A jar of water sits still on the counter, with small balls settled at the bottom. When caffeine enters the jar, the balls start bouncing up and floating near the top. The balls are brain cells, and caffeine is the push that lifts them into an active state. More balls near the top means more alert brain activity.
Explaining caffeine by grade level
Coffee has caffeine in it. Caffeine wakes up your brain. That is why some people feel more alert after a cup of coffee. They can focus and hit a target, like a dart board, more often.