Welcome to Day Two of “I Did Not Know That.” On Monday, we covered mispronunciations. Today, we explore so-called common knowledge that for whatever reason — we were absent that day, we didn’t pay attention to our parents — never sank in.

Elinor Stillman said she’s always been slow at unscrewing things, whether they were light bulbs or jar lids. “I had to stop and ponder which way to turn them,” she wrote. She mentioned this at a family gathering, and several of the men immediately said, “Righty, tighty; lefty, loosey!”

Wrote Elinor: “I had somehow gone past the age of 60 without ever knowing about this handy reminder.”

When Munam Goodwin first moved to the District at 20, she saw numerous signs that read: “No Standing. No Stopping.” This troubled Munam, who took the Metrobus to and from work and was fearful of running afoul of the law as she waited for the bus.

“So, I would pace back and forth to make sure that I was not standing and I was not stopping,” she wrote. “Years later I found out what it meant.”

A reader named Andrea said she was raised in Upstate New York and didn’t visit Washington until she was 21. Growing up, she would hear about artifacts that were being put “in the Smithsonian.”

Wrote Andrea: “I had always thought that the Smithsonian was that tall pointy thing that I had seen pictures of in textbooks — you know, the Washington Monument — and wondered how it all fit in there. I’m not sure when I was disabused of this idea, but to this day I almost always accidentally call the Washington Monument the ‘Smithsonian’ in my head before pointing it out to people — and identifying it correctly — out loud.”

A reader I’ll call “Jan” has had her driver’s license for 35 years. “I understand how cars work and am a good driver,” she wrote. “However, I only learned from my husband a few years ago what those little white lights on the rear of a car were: the back up lights. . . . I keep wondering what else I don’t know.”

Frederick’s Annie Hughes confessed that until about five years ago, she did not know that thunder is the sound lightning makes. “I am very embarrassed to admit that fact,” she wrote.

When Silver Spring’s Lisa Wheeler was 10-years-old or so an elderly relative passed away. Lisa’s mother told her that the custom was to go to the viewing, where people would view the body and pay their respects. Lisa freaked out. To her, a body was just that: a person minus the head.

“My mom wrapped her arms around me and explained that in this case, the body meant everything, head included,” Lisa wrote. “But, as she held me tight, I could feel her shaking with silent laughter, and I was mortified by my stupidity.”

Laurel’s Charlie Goedeke calls himself “a highly trained modern engineer” who has always enjoyed classical music. “For years I listened to and appreciated the music of Chopin — as in ‘Chopping,’ with a silent g — on recordings,” Charlie wrote. “At the same time I was vaguely aware of the existence of another composer, ‘Showpan,’ heard often on the radio. It wasn’t until I was in my 40s, when a friend was asked to play a Chopin piece on our piano, that the connection between the two finally clicked.”

Annandale’s Jane Pacelli said that she was baffled for years by two words that seemed to have similar, if not identical, meanings: “The word ‘subtle’ (presumably pronounced SUB-tul) was often seen in print but never heard in conversation,” she wrote. “Its twin was pronounced SUTT-el (and presumably spelled ‘suttle’) and never seen in print.”

Once when a Bethesda reader named Dorothy was in college, she ran into her philosophy professor at the bookstore. As a philosophy major, she was delighted to discuss metaphysical issues with him while perusing the store together.

“As we discussed existentialism, he pointed to a book by his favorite philosopher, Simone De Beauvoir,” Dorothy wrote. “Anxious to impress, confident in my superior knowledge, and before he could say a thing more, I piped up by expressing how much I loved his work and that I had enjoyed many of his books.”

One day when he was in his 10th-grade biology class, Germantown’s Vince Opperman listened to the teacher answer a student’s questions about blood transfusions and how important our RH factor is when getting blood.

“This seemed odd to me, not having done my homework,” Vince wrote. “So I asked: ‘What does our age have to do with it?’ Brought down the house.”

Tomorrow: More housebreaking mistakes.

For previous columns, go to washingtonpost.com/johnkelly.