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If you live in a city with modern sanitation, it's hard to imagine daily life being permeated with the suffocating stench of human excrement. For that, we have a number of people to thank - not least a London watchmaker called Alexander Cumming. Cumming's world-changing invention owed nothing to precision engineering. In 1775, he patented the S-bend. It was a bit of pipe with a curve in it and it became the missing ingredient to create the flushing toilet - and, with it, public sanitation as we know it. Roll-out was slow, but it was a vision of how public sanitation could be - clean, and smell-free - if only government would fund it. More than two centuries later, two and a half billion people still remain without improved sanitation, and improved sanitation itself is a low bar. We still haven't reliably managed to solve the problem of collective action - of getting those who exercise power or have responsibility to organise themselves.

Presenter: Tim Harford
Producer: Ben Crighton.