The best analogy I've heard is this:

Guy jumps out of a plane and plummets a thousand feet. Finally his parachute opens at 500 feet. He looks up and says, "Hey the parachute slowed my fall. I guess I don't need it anymore" and then cuts it loose.

And this is mine:
Guy slathers up with sunblock and goes out in the sun all day. That evening he says, "Hey, I didn't get sunburned, so I guess I don't really need sunblock, when I go out in the sun all day tomorrow."