Give each student in the class a length of string to represent a nuclear membrane and two (or more) pairs of pipe cleaners. Two individual pipe cleaners represent two chromosomes in normal condition; the second set of two can each be twisted together with each of the first pair to represent chromosomes in duplicated condition. Using a suitable diagram on an overhead projector or chalkboard, have students manipulate the "chromosomes" through the various stages of mitosis. Something about actually going through the motions of manipulating cell parts in this way helps students remember the stages of mitosis.