Oh I just remembered this while reading OP's story again.
I was in 2nd grade, and we were all sitting cross legged on the floor cutting and making cards for something. For some reason I flexed my richard and it did that up-and-down thing. I kept doing that and I'm like "Whoa!", literally out loud. Then everyone in our little mini circle looks at me, then looks down. One girl described it, "It's like its breathing OGM!"