I always start this lesson with the simple folding paper and cutting a half heart shape and let the kids go, "oh yeah, I already know how to do this...yada yada yada". (They think they know everything, don't they?) Then I tell them to throw the heart away, and that really gets them confused :) I go through the whole positive space/negative space speech and tell them we will be filling in the "window" (or, the negative space) with shredded paper strips. I love the effect, it reminds me of a paper weaving, but it is WAY simpler - if that is even a word??