You consume caffeine and your stomach lining mistakenly registers it as harmless, or food, the lower intestine imports the trimethylxanthine, the brain registers the oversupply of Adenosine (via the mistaken impostor of 1,3,7 trimethylxanthine) as some kind of neurotransmitter imbalance and emergency attack [fight or flight, back when you were a flatworm to swim away], to give an adrenaline to be smart now and get away from the disrupting-predator, which is why it makes your digestive system go funny peeing and pooping. When you hold a small wild animal like a toad or large bug, it pees on you. The human body has 0, 1 or 2 unique enzyme mechanisms that operate in the blood that can break down Caffeine and 1,3,7 trimethylxanthine and pluck those impostors off your adenosine receptors, and the speed of that process determines how hard the double-shot espresso punches you in the brain. I'm typing this from the perspective of someone who has 0 of those enzymes, not 1 or 2 that you may have.

https://www.youtube.com/watch?v=hmtQPrH-gC4