(Reposting this old piece so I can more easily find it. One line tweaked to make it generic.)

I found a fragrant pebble;
When I smelt it, out he came.
He turns quite green with envy
If left out in the rain.

I could trade him in for silver
Or beat him ’til he’s thin;
Reduce him to a third
if I boiled him in a tin;

But cruelty’s corrosive
So I treat him as a friend,
In hopes that I’ll be hearing
A purr there at the end.