Thomas Edison tried two thousand different materials in search of a 
filament for the light bulb. When none worked satisfactorily, his 
assistant complained, “All our work is in vain. We have learned 
nothing.”
Edison replied very confidently, “Oh, we have come a 
long way and we have learned a lot. We know that there are two thousand 
elements which we cannot use to make a good light bulb.”
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