It wasn’t one sip. I would have let that go. It was sip after sip after sip. So I turned to the guy sitting next to me downing his coffee on a Red Line train to Shady Grove, and as politely as I could manage, I said, “Hey, maybe you didn’t know this, but you can get fined for drinking on the Metro.”

Perhaps expecting a thank-you for the warning was too much, but I definitely didn’t deserve the look of disdain I got. That’s why I couldn’t stop myself from following up with, “Because it can spill and stain and attract rats, and that’s just not cool.” Apparently, questioning this 20-something’s cool touched a nerve. “I never spill,” he replied, and then took another sip.

That would have been the ideal moment to give him a sharp jab to prove that his luck could change, but as a transit rider who appreciates a clean car, I kept my elbows to myself. And instead, I’m dedicating this column to him and his ilk.

A lot of people break the rules against eating and drinking on Metro, but no one bugs me as much as the smug mug holders. The tourists who do it are ignorant. The teenagers who do it are insolent (or, in other words, teenagers). But the commuters who caffeinate every morning during rush hour should know better.

They set a bad example not just for tourists and teenagers, but also for the rest of us. See enough people taking hits of Starbucks and you’ll want to do it, too. It’s not that we don’t crave an occasional beverage on the train, but we abstain in the name of the greater good. All of these sip, sip, sippers wear down our willpower.

The most irritating thing is that they don’t seem to recognize that they’re doing anything wrong at all. Most snackers have the decency to sneak around, but the coffee clutchers brazenly break the rules as if there were different laws when it comes to lattes.

I worry I’m sounding like a scold. I recognize that some people have epic journeys and get hungry and thirsty. In the summer months, drinking water can be a matter of safety. (And it’s way less gross than the sweat everyone else is leaving on their seats.) That’s law-breaking I can get behind.

But do you really need that coffee to live? Some people would say yes. I say shut your mouth — and your lid.