Caffeine is structurally similar to adenosine, found in our brains. Both molecules are water and fat soluble so they easily cross the blood-brain barrier. In the brain, adenosine protects us by slowing nerve cell activity. Due to its similar structure, caffeine binds to the adenosine receptors. Caffeine therefore, not only blocks adenosine’s ability to slow nerve activity, but it increases nerve activity, leaving us stimulated, more alert, energetic, and occasionally with coffee jitters.