“No problem.”

That is how I used to reply to people who thanked me for holding the door open for them or for offering some other small gesture.

I don’t know where I got into the habit of saying this to people, but I do not say it anymore.

My mannerly response of choice now is the one my mother taught me over and over again as a child: “You’re welcome.”

You may see no difference between the modern, slangy reply “no problem” and “you’re welcome,” but there is one, slight though it may be.

So why not make the change back to the traditional response?

After all, any time you are more polite to your fellow human beings you spread the desire for them to be more polite to others.

Politeness is infectious — almost as infectious as rudeness is.

Consider: If someone cuts you off in traffic, then gives you a very rude gesture with his middle finger, are you not filled with instant anger and aggression?

Are you then more likely to be rude to some other stranger?

“Incivility is a virus,” says Christine Porath, a Georgetown professor and author of “Mastering Civility: A Manifesto for the Workplace.”

She explains to NBC News that rude reactions tend to create more rude behavior, creating a big negative spiral and a negative culture.

Being mannerly is especially important now. We live in a time of increasing rudeness — thanks in no small part to the way we treat each other on social media.

Social media has resulted in more group-think — whereby we are certain that we and our friends are 100% correct about any particular issue and those with whom we disagree are not only wrong but are stupid and evil.

Group-think has given us license to lash out at others without restraint.

Thirty years ago if you said some of the things to a stranger that some people say all the time to others on Facebook, you’d likely get punched in the face.

But the days in which there were consequences for such rudeness are long over.

Psychology Today says there is one key reason why people are so much ruder today: a lack of eye contact.

We behave differently hiding behind a fictitious online name when we do not have to make face-to-face contact with whomever we are verbally criticizing or attacking.

One solution: let’s be more mindful of being mannerly — online and off.

As a kid, I had it drummed into my head to say “please” and “thank you.”

Now, when I phone the electric company or a client, I always ask, “How are you today?”

It throws people off. Most of the time, they reply, “I’m great. How are you?”

And off we go, with a touch of civility established, to tend to our business.

I have to work hard at being polite — particularly in traffic — because I do have a temper and I do respond with aggression if someone gives me the middle-finger gesture after cutting me off.

A better way to respond, my wise mother keeps telling me, is to smile and wave — gestures that suggest “my bad” even though I did nothing wrong.

Truth be told, I’ve only managed to do this once. But boy, does it instantly confuse and disarm rude people.

Polite behavior always does.

Tom Purcell, creator of the infotainment site ThurbersTail.com, is a Pittsburgh Tribune-Review humor columnist. Email him at Tom@TomPurcell.com