Retrospectacle: A Neuroscience Blog

Debunking the Magic Toothpick Trick

First, watch this:

I saw that on Yahoo last night and thought it would be kinda fun to try to figure out how it was done. Here’s my guess. So, first of all, it seems that there are 5 toothpicks, broken in half (but not completely in half). The wood has been compressed due to the bending of the toothpick. Dropping the water on the toothpicks causes the wood fibers to expand as it takes up the moisture. Soon the bent toothpicks are pressing against their neighbors, which pushes them outward forming the star. Seem right?

Comments

  1. #1 Lazy
    December 3, 2006

    No idea if you are right but you are one gorgeous woman.

  2. #2 Ray
    December 3, 2006

    First, you are not trying to “debunk” it as much as to “explain” this (assuming the video is not trick photography). Your theory doesn’t seem right. Mere expansion would not bring the sticks together in the first part of the “trick.”

    I would say that toothpick pieces first drawing closer to each other (to form the 5-arm star) is probably due to surface tension of the water. We can’t see how much water there is because of the quality of the video, but it might be that the there is enough water to creep up between any two pieces. This draws them together.

    The second part where the center then expands is hard to explain and I have to theories on that. But I suspect the wood fibers being able to absorb water might have something to do with it.

    The best things is to find wooden toothpicks and try to repeat the experiment!

  3. #3 Roy
    December 3, 2006

    No, I’m pretty sure she’s got it right, Ray.
    Watch the first part again- you’re thinking of them as getting closer, but it’s really the broken toothpicks straightening back out, and pushing up against their neighbors. See how the “V” shape opens up?

  4. #4 shelley
    December 3, 2006

    I just tried it with the broken toothpicks and it *kinda* worked–not as good as in the movie but enough that proof of concept seems reasonable. Hot water worked better than cold btw……anyone else try it?

  5. #5 Crudely Wrott
    December 3, 2006

    Shelly, you are spot on.

    My Dad taught us this trick back in the late 50s. The first time I watched it I got that “spooky feeling” that one commonly feels when seeing something unknown for the first time. I certainly didn’t know how it worked but was certain that I could discover why.

    I already knew (at age 7) that wood will absorb water and expand, especially dried, well seasoned wood, which toothpicks are. I also knew that my Dad would do things like this to encourage me and my sibs to think and use reason to solve a puzzle.

    I found that some things were critical to obtaining a well-proportioned star. First, the choice of the toothpicks (round ones, please) – length and thickness should be similar. Toothpicks of a slightly darker shade were denser and reacted slower. The break needed to be carefully done – any toothpick that splintered or broke too much would spoil the symmetry. The positioning of the broken picks is touchy – they should create a very round circle in the center about a quarter inch in diameter. And most importantly, a single drop of water in all that is required. Any more tends to float the picks and cause them to move prematurely. Oh, yes. A smooth, flat and waterproof surface in required. A formica tabletop is ideal.

    You are correct in seeing that it is the swelling of the unbroken wood fibers that causes the effect. Consider what wood is made of: lots and lots of capillary tubes arranged in parallel which compromise the grain of the wood. After all, wood, and other plant fibers have one chief function, the transport of fluid. So moisture will naturally wick into the grain. When this happens the individual fibers expand across their diameter, which is what causes a board to expand in width perpendicular to the grain.

    When you break the pick across half of its thickness and bend it into a V shape their is a hinge of unbroken fibers on the inside of the V. Add water, the fibers in the hinge expand and, Presto! Instant motion. It should be noted that you needn’t use five picks. Other numbers will work, resulting in other geometric shapes.

    This is a great little demonstration to provoke a wide range of reaction from people. Kids love and so do grown ups. It “tickles the brain,” as Dad used to say. Thanks for the post.