Used to be that when a play ended, the audience would rush the stage to pat the backs of the actors to show appreciation. After the Great Audience Stampede of 1733, they made a rule to not rush the stage and just use the other hand as a substitute of the actors’ backs. That rule became the social norm.
I remember when I Ran by Flock of Seagulls was randomly playing in my head so I decided to listen to it. YouTube took that as me finding a new favourite band. I mostly listen to industrial metal.