Why does caffeine give you so much energy, while being so low on calories?
I agree with @inf3rno: caffeine is a stimulant that acts on the brain and various other parts of the body (Snyder et al., 1981) and I wish to elaborate on its psychopharmacology. Caffeine's effects in the brain are mediated through adenosine A1 and A2 receptors (Daly et al., 1983). This results in a variety of actions (Fredholm et al., 1999). Most notably, adenosine inhibits neurotransmitter release in various brain structures including the acetylcholine release in the mesopontine projection neurons. Caffeine blocks these adenosine receptors, in turn leading to an increase in acetylcholine release in the cortex and hippocampus. This increases vigilance and information processing. Increased acetylcholine release in the prefrontal cortex may lead to increased attentional abilities. Adenosine also inhibits the effects of dopamine released by the striatium, ultimately leading to disinhibition of motor output in the brainstem. This in turn increases motor activity (Fisone et al., 1997). In all, caffeine does not exert its stimulant effects through its metabolic energy content, but due to its pharmacologic effects on adenosine receptors.