Louis Meyer became the first three-time winner of the Indy 500 (1928, 1933, 1936) when he won the 24th International 500-Mile Sweepstakes Race held at the Indianapolis Motor Speedway on Saturday, May 30, 1936. He regularly drank buttermilk to refresh himself on a hot day and happened to drink some in Victory Lane as a matter of habit after winning the 1936 race.
It is reported that an executive from the Milk Foundation was so elated when he saw the moment captured in a photograph in the sports section of his newspaper the following morning that he vowed to make sure it would be repeated in coming years.
There was a period between 1947-55 when milk was apparently no longer offered, but the practice was revived in 1956 and has been a tradition ever since.

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