You're standing outside a friend's car. You want him to roll down his window so you can talk.
If you're from my generation or one earlier, you make a rotating, cranking motion with your hand, simulating the way that windows were once rolled down before power windows.
This, to me, is universal sign language for, "Roll your window down, please."
In truth, car windows don't actually roll down. They never have. They are simply lowered into a space within the door. It was the rolling motion of the manual crank with led to the phrase "roll down the window."
My children have never seen a manual crank on a car door. Unless they see one in an old movie, they may never see one. It's likely that they will never crank a window down in their lives.
All they have ever seen is a small button on the car door. Depress it slightly and the corresponding window is lowered.
So years from now, Clara or Charlie is standing beside a car. A friend is inside. They want the friend to roll down the window to talk.
What will they do? What motion will they make with their hands to signal the desire to open up the space between them and their friend?
These are the things that keep a parent up at night.