Caffeinated means containing caffeine, a natural chemical that makes you feel more awake and alert. When a drink is caffeinated, it has caffeine in it: coffee, black tea, and many sodas are caffeinated beverages. Energy drinks are often highly caffeinated, meaning they contain a lot of caffeine.

Caffeine works by speeding up your brain and body slightly. That's why adults often drink caffeinated coffee in the morning to help them wake up, or why someone might choose a caffeinated soda in the afternoon when they're feeling sluggish. The opposite is decaffeinated (or decaf for short), meaning the caffeine has been removed.

People also use caffeinated informally to describe someone acting unusually energetic or jittery, as if they've had too much caffeine. If your friend is bouncing around the classroom full of energy, you might joke that they seem “really caffeinated today,” even if they haven't actually had any caffeine at all.

Most doctors recommend that kids avoid or limit caffeinated drinks because developing bodies and brains are more sensitive to caffeine's effects. Too much caffeine can make it hard to sleep, cause headaches, or make you feel shaky or anxious.