“The world's most popular drug! (the buzz most enjoyed around the world)”
Neuroscience Resources for Kids – Caffeine
Neuroscience Resources for Kids – Caffeine — A nice set of web pages that explain many aspects of neuroscience, including, in this case, caffeine.
What’s your poison: Caffeine
Caffeine’s uses and side effects — from WebMD.
Caffeine fact sheet
Caffeine — from the National Institutes of Health MedlinePlus.
Caffeine, the good, the bad and the maybe
tl;dr: The world's most popular drug. Beware the withdrawal; it can sneak up on you.