What childhood lesson has always stuck with you?

There are two - both of them, naturally, come from my father.
The first has to do with driving safely, and once it is in the back of your mind, you'll never forget it.
He said to me "Whenever you are driving in the evening, and you wonder to yourself 'Should I turn my headlights on?'...you should. That is the exact moment when you need to use your headlights."
This lesson is semi-irrelevant for people who drive cars with daytime-running-lights, but even those individuals will need to see their dash (speedometer, odometer, etc).

It has never left my mind, for now whenever I am driving at dusk, and I think "is it dark enough to turn on my headlights?"... I know it is time to do exactly that.

The other is about gratitude, courtesy, and honorable behavior. It is extremely simple, but it has always stuck with me.
When I was growing up, my parents would occasionally find the time and money to take my sister and I on a 'vacation', though we were poor and it never resembled most extended/overseas/Disney type vacations that other families often took.
But when we could, we would all pile into a car, pack up some gear, and head 2+ hours out of town to go to the beach or (more often) camping.

Because of the our limited means, my parents often had older and unreliable cars. Sometimes they were so mechanically questionable that we didn't trust them enough to even take them 70, 80 miles out of town.
So my grandparents would offer us the use of one of their vehicles (at times they had access to up to 3 cars between the two of them: their cars, and a company car on loan).

Whenever we would return from our 2 or 3 day R&R, I distinctly remember my Dad making sure to carefully clean both the exterior and interior of the borrowed car, essentially "detailing" it, as it were.
Even though the car had not been immaculate upon receipt, even though our trip hadn't caused too much (or any) grime to accumulate within or without the vehicle.

And when asking him "Why are you cleaning it so much?", he would pause and tell me, very firmly:
"My father always told me that whenever you borrow something, you should always return it better than you received it."

Like paying "interest" on a loan of kindness.
And it's true. It's really true. In some way or another, you really should.

when I make Evan laugh - it is as though a match is struck in the furnace of my very soul—a soul that had, until now, cast only a pale anemic light in the cave of some peaceable but ordinary life.
But now... he laughs and it fills the universe and the sun itself is a whimpering ridiculous flame in comparison.