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The ALS ice bucket challenge has become a viral phenomenon. People around the world have been dousing themselves in ice-cold water and in the process have raised over $100m for charity. But a true nerd doesn't run with the herd, and Tim Harford is only going to do the challenge if the facts stack up. He investigates whether a viral challenge like this is good for charitable giving overall, and whether there are reasons to be more choosy about the charities we give to.

(Image: KISS ALS Ice Bucket Challenge. Credit: Getty Images)