In 1928 Walt Disney's team wanted a cartoon whose sounds seemed to come from the drawings, and nobody had ever seen a drawing make a noise. They tested it behind a bedsheet: Wilfred Jackson played harmonica and a spittoon was the gong. But how do you draw to a tune nobody has played yet? Jackson's mother taught piano, so he brought in her metronome. Film ran ninety feet a minute, so at 120 ticks a minute every tick is twelve frames, and the whole cartoon went onto a chart with a square for every beat. In New York on 15 September 1928, seventeen musicians played it, and it failed: the players only had the picture to follow. Walt later said they drew a mark on the film every twelve frames that flashed on screen for the conductor. (Disney's archivist describes it as a bouncing ball.) The re-recording worked. Variety called Steamboat Willie not the first sound cartoon, but the first that people liked.

The metronome, the filmstrip and the little boat here are our own drawings, not footage from the film.

Which movie trick should we take apart next?

Sources: Wilfred Jackson interviewed by Michael Barrier (1973) and in Funnyworld 13 (1971); Dave Smith, Disney archivist, Library of Congress essay on Steamboat Willie; Walt Disney in The Story of Walt Disney (1956); Variety, 21 November 1928.
Sounds: Freesound, CC0 (Bidone, Druminfected, videofueralle); Apple iMovie sound effects.