When you're drunk/high/whatever certain senses start to fail you... particularly when you turn all the lights off... so you begin to optimize certain functions. Like telling if a beer is empty or not. Chances are you'll pick up a beer and need to know 'is this empty.' You'll pick it up, probably swirl it around a bit, but you know what... that doesn't tell you anything you didn't already know. Your a drunk. You knew as soon as you picked it up just how full/empty it was; everything else was just muscle memory... and that's how you keep from spilling beer when your too fucked up to know what senses to trust.