The Nine Dot Puzzle

Puzzle: using 4 straight lines, connect all nine dots without breaking the line (i.e. lifting your pen off the paper. Don't do write on your computer monitor, mmkay?). Easy? Can you do it with 3 straight lines?

Display solution: click for the solution - but give it a try first!

See solution (but give it a try first!)

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My friend did it by simply back tracking to form the letter E. There is nothing that say you cannot backrack. You cannot cross anothe line, which
did not.
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As an IBMer, I presented this problem multiple times for 3 separate years 2002-2005 at Disney World for engineering week. I presented all of the solutions stated in prior comments. The intent of the different solutions is not just to think outside the box, but also show that there is more than one right answer, as well as to show that different people have different boxes. Math people think dots and lines with width is "cheating". However, scientists working with a physical world have a box where this is "normal". Moms with small kids have no problem imagining a fat crayon like pencil which is fatter than the picture of all 9 dots... ditto for painters Computer scientists have no problem with regular pencils, but microdots... becoming increasingly smaller until the dots seem to converge. Other engineers do a variation of the "around the world" solution. The pencil is stationary and the paper is rolled onto a cylinder, with the cylinder rolling. The pencil's straight line indeed can cover the entire paper.

The 4 and 3 line solutions were documented in books. I had not found the other solutions documented elsewhere prior to using them in the presentation. I'm glad to see these other solutions now being written and used.
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Is there an author / creator name to the nine dot puzzle that should be credited in a publication? I have used it for years and plan to publish it as an example of thinking outside the box.

PS I will gladly mention your website for more puzzles if appropriate.
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