Help finding a poem

Hello, I'm trying to find a poem I read somewhere once and I'm not having any luck with any search engines. I've also tried chatgpt and it keeps suggesting different poems.

What I remember about the poem: I think the stanzas begin with "I sometimes think..." and the poem ends with something like "in fact I do little but lie on my back, but I sometimes think." Also, it rhymes zinc with think, something like "or study the crystal structure of zinc"

edit: the end is something like this:

I sometimes think I should learn to play the sackbut

or study the crystal structure of zinc

In fact I do little but lie on my back but

I sometimes think