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A travelling parent with kids cannot take a 460-millilitre jar of peanut butter through airport security. It’s not an allergen issue, but rather it is too large: a spreadable food, treated like a liquid. But any number of containers of it less than 100 ml is OK. At one airport, a blazer comes off; at another, it stays on. The same thing with shoes and laptops, all according to the rules that can look arbitrary or capricious to onlookers. For Margaret Bloodworth, the former deputy minister of transport who helped manage Canada’s response on Sept. 11, 2001, that apparent regulatory...