Skip The Water

Skip The Water
avc.com

There was a Japanese TV program a few years ago testing talented youngsters. One brain teaser was "On this table you have six match sticks of equal length. Arrange them such that you create four triangles."

The match sticks are presented lying flat on a table, and approximately everyone tries to solve the problem in 2D and fails hideously. The right answer, which is a peculiar kind of perverse clever that math teams and startups are fond of, is to make a triangle on the plane and then point the last three match heads upwards from the vertices, creating a pyramid. Each of its four faces is a triangle. Half of the wunderkids got this right.

One kid ran out of time and didn't get it. After being shown the 3D solution, he skipped the next problem entirely and would only say "It works in 2D, it works in 2D, I know it works in 2D" and continue playing with his match sticks.

He eventually solved it halfway through the next problem, confounding the host because his solution was not written in the notes: make two triangles in the plane, flip one, and arrange them to form a Star of David. (Nobody here would call it that, but I'm assuming most HNers know what one looks like.)

Quote the kid: You said I had to make four triangles, but you never said anything about the fifth, sixth, seventh, or eighth.

And a transcript for those who want to skip the video: http://dotsub.com/view/e358ac0c-314e-473a-b051-f0a2deaa3a7b/...

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.