"Using only paint stirrers and 3/4" washers, build the longest cantilever span you can. You can use as many washers and stirrers as you want. Go"
(Okay, even with $65 worth of washers, we ran out of materials for all of our engineers. Our solution was to allow our engineers to substitute World Book Encyclopedias for anchor weights, When we extend the challenge to the longest span with the most efficient amount of resources, students may exchange washers for an encyclopedia- approximately 27 washers per volume).
One rule we have? Cheer when a design collapses- since we've learned something very important. We encourage students to "fail early, fail often" in order to promote exploration and "thinking outside of the box."