Something unfortunate occurred after I opened my mouth to save the earth. It was a T-shirt-temperature Brooklyn night, and I was loitering outside a subway entrance, waiting for friends. While commuters marched out like ants, a single-toothed man shook a creased, change-filled Dunkin’ Donuts cup like a maraca.

“Spare a quarter ... spare a quarter ... ” he repeated, his pleas accompanied by tinkling nickels and pennies. It was a sad, lonely song repeated, endlessly, like an MTV “Real World” marathon.

Most after-workers switched on invisibility rays and scurried past. I lacked such superpowers. In a lull between train arrivals, Mr. Cup stumbled near. He slurped a can of Coke. I flashed my awkward white-toothed smile. Then he tossed his can on the sidewalk and belched.

Oh no he didn’t. I can tolerate my neighborhood dealers selling dirt-brown weed. I can tolerate booty-bass low riders cruising my block. I can even tolerate Ann Coulter. But littering? Nu-uh. It’s pure laziness, each casually tossed candy wrapper a filthy, disrespectful slap to the landscape. Besides, this was five-cent-deposit New York City. A trip to the local recycling center saves the environment and garners a handsome nickel.

Like a helpful Nellie, I said, “Excuse me, sir, but you dropped your can.” After all, maybe his eyesight was failing. Ground? Garbage can?

“You don’t like it, pick it up yourself,” he said, revealing newborn-smooth gums. He shook his change cup with renewed fervor.

I should have dropped my complaint just like his can. This was the big, bad Big Apple. Each stranger is a potential nutcase who power-saws humans, just like a recent subway psycho. But this is blessed hindsight talking. In the impetuous, pressure-cooker moment, I said, like a ’roid-raging Miss Manners, “What, who’s going to pick up after you? Your mom?”

“What’d you say? What’d you say about my moma? You don’t know anything about me and my mama. We go back. We go way back. We have His. Toe. Ree,” he said, punctuating every syllable. “His. Toe. Ree. His. Toe. Ree.”

He lumbered forward, standing nose to nose with me. His bloodshot eyes were red, red like a fire engine.

“I wasn’t talking about your mom,” I said, inching back. “I wanted you to think about the environment.”

“No, no; you were talkin’ about my mama. You know nothin’ about her, man. How would you like it if I talked about your mama?”

Mama, dad, cousin, kielbasa: My words, any words, were kindling igniting dry brush. “I was talking about the Coke can, sir.” A little deference might soothe his ire.

“Why are you talkin’ about my mama?” Mr. Cup repeated, mastering the fine art of subject changing. “Apologize,” he said. “Apologize.”

Stubbornness is my family’s curse. My grandfather died because he refused to visit a doctor as lymphatic cancer ate his insides. I take after him.

“For what?” I asked. “You littering?”

“No, about my mama.”

“I wasn’t even talking about your mom.”

“Yes you were!” Like a misguided lion, he roared, threatening to insert my head in a dark, smelly, sunshine-deprived hole. “And I have nothin’ to lose,” he added, his breathing ragged. “Nothin’ to lose.”

He was wrong yet again. Small kernels of environmental disobedience add up. Coke cans today, toxic waste tomorrow. We’re all stuck on this rotating rock, and it’s our ingrained duty to keep it pristine. Pack it in, pack it out, in life and death. Simple, Smoky the Bear logic, right?

But this situation long sped past college-educated reason, and my bachelor-degree smarts lacked street-fighting skills. Eyeball to eyeball, we were caught in an erroneous Mexican standoff. Would he recycle? Would I recycle? Would he shove my skull through a plate-glass window, which could not be recycled?

Our ecological quandary went unanswered; beneath my quaking feet, the ground rumbled. A train! Weary passengers soon slogged past. Ever industrious, Mr. Cup returned to work and, thankfully, invisibility.

My friends soon arrived. We quick-stepped into the night, leaving the Coke can on the sidewalk like a monument to a near disaster. Only later on, as I lay in bed too wound up to sleep, did the light bulb click on: I was a self-righteous, knee-jerk idiot. Mr. Cup didn’t need my meddlesome advice. He understood environmental principles to a T, employing conservation tactics that most overlooked.

After all, would just any eco-warrior reuse that Dunkin’ Donuts cup?