Buster Keaton Tries, Fails to Make Milkshakes
In the 1927 silent film College, Buster Keaton plays a college student who takes various jobs while trying to woo a classmate. My favorite sequence in the movie is when he takes a job as a soda jerk. It's a classic comedy setup: We see a guy who's actually good at the job, and then we see Buster Keaton doing his best at it.
For a film that's 88 years old, this really holds up. I love Keaton's tiny eye movements as he works through the gags.
You can watch or download the entire public domain film for free, or read an excellent writeup of it for more.