When pulling your speed boat out of the lake with your '69 Roadrunner 4-speed, don't let your drunken/stoned friend drive the car up the ramp if he doesn't know the reverse indicator light means the car is in reverse when he let's the clutch out. "Don't ask my friend Bill how he knows..."

There is a reason for wearing welding goggles when welding. "Don't ask my friend Steve how he knows..."