You get a hangover because your brain is mostly made up of water. When you drink and get a hangover you become severly dehydrated. Becuase of the relatively short amount of time that this sudden loss of hydration occurs it'* a shock to the brain. The brain literally shrinks inside of your skull and pulls against the tiny string-like membranes that hold your brain to your skull. WHen this happens, you get your tremendous headache. This is also why it hurts to move your head around. Your brain is now smaller from the lack of water and can move around more inside your head.