The mayor of Philadelphia, site of the 150th meeting of the American Association for the Advancement of Science, stunned nutritionists in his welcoming speech by telling the assembled audience of the pedigree of the city's food - including its infamous cheesesteak. Only in Philadelphia can you get the unique combination of poor quality meat - providing the right kind of grease - processed cheese and greasy onions needed for this delight, the mayor claimed. His speech followed hot on the heels of a press conference on obesity, which suggests that all too many Americans share his honour's culinary taste.