What counts as a summer Olympic sport? It depends on the year. In 1896, freestyle swimming for sailors was on the lineup. In 1920, tug-of-war. Live pigeon shooting was a new event in 1900 (like 184 other events through the years, it lasted for only one Olympiad). With a few enduring exceptions—the 1,500-meter swim and the shot put come to mind—individual events and even whole categories go in and out of fashion: This year women will enter the boxing ring for the first time, and tennis will gain a mixed doubles event. Here’s a look at how long each Olympic category has been able to keep the flame burning.