In a similar class, we were given the problem of designing a large yo-yo. The judgement criterion was the diameter multiplied by the return ratio (how far back

In a similar class, we were given the problem of designing a large yo-yo. The judgement criterion was the diameter multiplied by the return ratio (how far back up it rebounded). There was no hard-and-fast definition of a yo-yo. Experimenting with various designs, my buddy and I saw that the biggest challenge was preventing precession, which caused the string to rub against the disks, wasting energy and killing the return ratio. After plying the prof with leading questions about the design requirements, we submitted a yo-yo made with a single disk, cut from a fridge-sized cardboard box. It had two strings, one running down each side of the disk. That ended precession nicely. Our entry had both the largest diameter (we had to test it in the stairwell, because the doorframe used to test the other entries wasn't tall enough) and the highest return ratio (in the high-90%). The best part was that, in response to the predictable storm of objections from the other students, the prof turned it into an object lesson. He made the point that he had never stipulated there had to be one string and two disks, just that there had to be a disk to measure, and a string by which to calculate a return ratio.