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I RECENTLY received an email that began, “Dear Dr Len, I do PR for a breakfast cereal company, and we would like an authoritative figure, such as yourself, to come up with an equation as to when to include the milk.”

I have been getting similar requests since 1998, when I agreed to provide a biscuit company with an equation to predict the optimum time to dunk a biscuit in a cup of tea. My motives were honourable – I hoped to make science accessible by showing how scientists think about the problems of everyday life. The equation was an…

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