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