On the first day of training for parachute jumping, a man listens intently to the instructor as he tells them to start preparing for landing when they’re at 100m.
“How am I supposed to know when I’m at 100m?” the man wants to know.
“That’s a good question,” the instructor says. “Well, when you get to 100m you can recognise the faces of people on the ground.”
The man thinks about this for a moment.
“Okay,” he says, “but what if there’s no one there I know?”