HELP WITH FORMS

A certain poetry class is studying forms. What's your favorite form? How does it work?
Pantoum
http://www.baymoon.com/~ariadne/form/pantoum.htm
Four lines in first quatrain. Second and Fourth line become first and third line of the next quatrain, and again for as many quatrains as you want. The last quatrain should repeat the first and third lines as second and fourth. Lines vary slightly, and that is fine.