I was sittin on a fence post, chewin my bubble gum (Smack, smack, smack),
playing my yoyo (doo-wop, doo-wop)
and along came Herman the worm,
and he was thissssss big
And I said, “Herman! What’s up with that?”
And he said, I just ate a worm

Herman eats something new and bigger after each verse, and that thing is pantomimed
Replace “thiissssss” with an acurate indication of how big Herman is based on what he is eating that verse
End with Herman getting small again because, “I just burped”