A bell that rings two notes at once. Explaining how a bell goes ring-gin-gin-gin (with the sound going up and down as the bell rings out). The key is multiple modes of vibration.
Expanding on this, I experimented with making a bell that sounds two distinct notes, forming roughly a major third.
It turns out numberphile did a video on this topic last year, but relating to coffee cups: https://youtu.be/MfzNJE4CK_s
Tags: A bell that rings two notes at once, physics, bell, vibrations, music, sound, interference, lissajous figures, peldulum