Ok, forgot the water, but hangovers aren't just caused by dehydration, although that is the major contributor.
People have noticed for years that darker drinks give worse hangovers than lighter drinks (i.e. whisky as opposed to gin) because of complex protein (i think) compounds called congeners. Also, combining congeners from different drinks tends to leave you feeling worse in the morning... hence the saying don't mix the grape and grain...
The burnt toast works because of the carbon in it which acts as a filter for the congeners.
p.s. coffee is not good for a hangover because of the diuretic effect
