Matt Grover - Freelance Camera Operator & Editor

Get yourself a whiteboard, draw a line on it with PERMANENT marker (yes permanent).

Let it dry.

Now draw over it with a normal DRY WIPE marker

Now wipe both lines away.

AMAZING!

NB you'll probably have to use some cleaner to properly clean your whitebaord, but at the end of the day I was pretty damn impressed with that here in the office!! :)