Hangovers only exist because it keeps us from partying and drinking and going nuts every single night. It's a fail-safe for humans so we don't all devolve into skinny blonde mid-twenties actresses at once. It's payment for fun. It sucks. But what's the real science behind it? It all boils down to three things.

Dehydration. Acetaldehyde. Congener. Damn those three things. Damn them in the morning. Damn them in the afternoon. Screw them at night. [SciShow via Viral Viral Videos]