In his pursuit for a filament for the light bulb, Thomas Edison explored almost 2,000 different materials. "All of our work is in vain," his assistant grumbled when none of them performed adequately. "Nothing has been learned."

"Oh, we've come a long way and we've learnt a lot," Edison said firmly. We now know that there are 2,000 elements that can't be used to build a good light bulb."

Moral: Take notes on what you've learned. You never fail; instead, you always learn.