I was heading home on the T the other day, gazing out of the window as the Green Line train pulled into Longwood. Across the tracks sat a young woman, presumably waiting for the next inbound train, which was 15 minutes away. As I watched, she reached into her handbag and took out what looked like a large sandwich wrapped in wax paper. She pulled off the wrapping, took a bite, and casually tossed the wax paper behind the bench on which she was sitting.

There was a trash barrel less than three feet away. She never glanced at it.

We live in a time beset by grave problems and noxious ills, most of which you and I can personally do nothing to cure: hurricanes and forest fires, gangrenous politics, global jihad, the Kardashians. But just because we can’t fix all the villainy and squalor in America doesn’t mean we can’t eliminate some of it. You want to make America a better place? I’ve got a few suggestions.

Starting with: Don’t litter.

If you wouldn’t fling garbage on your own private property, don’t fling it in our shared public property. Litterbugs are selfish jerks — all of them, without exception — and you don’t have to be Iron Eyes Cody to know it. You don’t even have to be a right-winger. Slobs who leave their coffee cups, scratch tickets, and empty cigarette packs on the sidewalk or park bench for someone else to clean up make the world more terrible. Refuse to be one of those slobs, and you’ll make the world more wonderful.

You know who else makes the world more terrible? People who deploy umbrellas large enough to shelter a family of eight, taking up the entire width of the sidewalk and leaving no room for anyone approaching from the other direction. They want to hoist an umbrella so wide it should have its own ZIP code, that’s their business. But when they use it to block or barge into other pedestrians, they become a public menace. It’s a matter of rudimentary courtesy: People carrying an umbrella are obliged to move or lift it out of the path of anyone not carrying one.

There is another evil that I have seen under the sun, and it lies heavy upon humankind: Hoggish customers who don’t return their shopping carts. The behavior is indefensible, and the offenders’ lame excuses — “It’s too far to walk.” “I have small children in the car.” “That’s the cart wrangler’s job.” — only compound their offense. Let’s be clear: If someone had no objection to pushing a cart up and down every aisle of the store, no complaint about wheeling it to the register, no reluctance about trundling it out the door and to a parked car, then refusing to return the cart to the store or to a corral in the parking lot is lazy self-centered wickedness. You want to bring some joy to this vale of tears? Put your shopping cart back where it belongs.

While on the topic of shopping, a word to people who “sample” grapes or cherries at the grocery store: Don’t. There’s nothing subtle or complicated here — taking fruit without paying for it is stealing. Claiming you want to make sure the fruit tastes OK before you buy it is an empty rationalization: Theft is theft. Shoppers are no more entitled to eat grapes they haven’t paid for than to drink a beer or consume a couple of bananas they haven’t paid for. When grocers want customers to sample food, they have an amazingly shrewd way of encouraging it: They put out free samples. Until you see a sign reading “Help yourself,” don’t help yourself.

Like Henry Higgins, I’m a very gentle man, even-tempered and good-natured who you never hear complain, who has the milk of human kindness by the quart in every vein. But let’s face it: Some people are a pestilence and shouldn’t be allowed out in public.

People, for example, who are so addicted to talking on their cell phones that they carry them into public toilets and never pause in their yakking. People who think a table in the middle of a busy Starbucks is a suitable venue for putting on makeup. People who drive down the street at full speed during a rainstorm, blithely plowing through puddles and drenching pedestrians. People who don’t clean up after their dog unless someone is watching them. People who berate cashiers or treat servers with disdain.

Don’t be those people. Why add to the world’s ample supply of annoyances? You only pass this way once. Might as well leave it a little brighter.

Jeff Jacoby is a columnist for The Boston Globe.