honey and sting

I'm winding up to teach my annual poetry-writing workshop, and thought I'd do a bit on form and how form informs/expresses content. A very *light* touch on the topic, as the workshop is mostly not didactic.

To that end, does anyone have a particular favorite of a particular poetic form, one that gives a sense of why that poem HAD to be a sonnet, or haiku, or whatever it is?

thanks muchly!