Dust-to-Digital posted a fascinating video in which a boy in a recording studio yelled into a paper cup that was connected to a spinning cardboard disc with a glass cup on top. Once the recording stopped, the engineer played the boy’s phrase back to him in perfect, although quiet, replication.
When he’s screaming into the top cup it’s vibrating a little needle that is etching a pattern into the glass. When this is being played back, the same needle is resting inside the groove and hits the sides of the jagged edge that the needle created, resulting in the cup vibrating, and making noise. This is exactly how a record works and is able to be played back.
This setup is similar to vintage wax cylinders that were played on a phonograph, which used the process of engraving to record sound.
LeoMakes posted a video showing how to make a microphone using a paper cup.