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. This programme was first broadcast on the BBC World Service.