User: What is the Indianapolis 500?
Assistant: The Indianapolis 500, formally known as the Indianapolis 500-Mile Race, and commonly called the Indy 500, is an annual automobile race held at Indianapolis Motor Speedway (IMS) in Speedway, Indiana, United States, an enclave suburb of Indianapolis. The event is traditionally held over Memorial Day weekend, usually the last weekend of May. It is contested as part of the IndyCar Series, the top level of American open-wheel car racing, a formula colloquially known as "Indy car racing". The track itself is nicknamed the "Brickyard", as the racing surface was paved in brick in the fall of 1909. One yard of brick remains exposed at the start/finish line. The event, billed as The Greatest Spectacle in Racing, is considered part of the Triple Crown of Motorsport along with the 24 Hours of Le Mans and the Monaco Grand Prix, with which it typically shares a date.

The inaugural race was held in 1911 and was won by Ray Harroun. The event celebrated its 100th anniversary in 2011, and the 100th running was held in 2016. The event was put on hiatus twice, from 1917 to 1918 due to World War I and from 1942 to 1945 due to World War II. In two different periods, the race was part of FIA World Championships; between 1925 and 1928, the World Manufacturers' Championship and between 1950 and 1960, the World Drivers' Championship.

Marcus Ericsson is the current champion. The most successful drivers are A. J. Foyt, Al Unser Sr., Rick Mears and Hélio Castroneves, each of whom has won the race four times. The active driver with the most victories is Hélio Castroneves. Rick Mears holds the record for most career pole positions with six. The most successful car owner is Roger Penske, owner of the Indianapolis Motor Speedway and Team Penske, which has 18 total wins and 18 poles. Penske also has five wins at the IndyCar Grand Prix, held on the combined road course.

The event is steeped in tradition, in pre-race ceremonies, post-race celebrations, and race procedure. The most noteworthy and most popular traditions are the 33-car field lining up three-wide for the start, the annual singing of the chorus of "Back Home Again in Indiana," and the victory lane bottle of milk. Also unique is that qualifying requires the driver to complete four, rather than one, timed laps. Qualifying has a separate weekend. The official attendance is not disclosed by Speedway management, but the permanent seating capacity is upwards of 250,000, and infield patrons raise the race-day attendance to approximately 300,000.