The most recognizable, these days, is the iPad pirouette, evoking an upturned palm. Gerard Knight led the design team at Square, one of the major tablet-payment providers, when it first rolled out its tipping feature. “Turning around the interface to say ‘Give me money’ can be kind of an obnoxious gesture,” he told me. Originally, the designers used a Trojan horse, of sorts. “The idea was you turned it around anyway, to capture a signature”—most credit cards at the time required one—“and in that process you prompt that customer for a tip.”