Do You Knock First or Just Enter?

My son stopped by after work the other day. He works really early hours and is off by about 9:00 a.m. He walked into the house, greeted my husband and I, and all the animals, grabbed a bowl of cereal and sat down at the dining table with his earbuds in, and ate. He had an appointment later in the morning and didn't want to drive all the way home just to turn around and come all the way back. That's cool.

I left for work shortly after that, but I realized as I drove away that I was grinning. I...