system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do hangovers work?

what is the science behind it? how can you get symptoms of sickness without actually being sick? why does your body respond that way?
gpt
Alcohol is poisonous. Filling your body with poison is generally not a good thing, so your liver has evolved to be able to convert that poisonous alcohol into harmless acetic acid (which can even be further broken down into carbon dioxide, releasing some useful energy in the process - this is why alcohol has calories). Unfortunately, there's an intermediate step here. Alcohol cannot be directly converted to acetic acid, it first has to be converted into something called acetaldehyde, which is *way* more poisonous than alcohol is. 

The steps your body goes through after drinking alcohol depends on the quantity of alcohol and acetaldehyde in your blood. When you're drinking, you have a lot of alcohol and not much acetaldehyde, so you get to experience the stuff alcohol does - the fun of your brain not working properly. During a hangover, most of that alcohol is now acetaldehyde, so alcohol is no longer inhibiting your brain's ability to tell itself not to do stuff (including not to feel pain), and instead you get a whole bunch of acetaldehyde toxicity, combined with the pains of dehydration, sleep deprivation and a range of other things that all individually suck and are all going on at the same time.