Consider 9 dots arranged in a three by three square. Connect all these 9 points with four lines without lifting your pen from the paper.
(In reply to
re: more difficult by levik)
method number 1:
This relies on the fact that the visual representation of a point is in fact not a point, it is a small solid circle. Therefore, you can draw a line through abc so it is slanted slightly down, and extends very far to the side, then double back so that the next line slants through fde the other way, and finally double back again to go through the last three. What this looks like is a very wide and flat "z", and of course only works on graphical points, not points as described in geometry.
method 2:
the standard trick with English (how many months have 28 days? - all of them, but some have more) It does not say only three lines. Simply draw a line through abc and extend a small distance. Draw another line which does not touch any points, but lines you up to go through fed, which you do with the third line, extending slightly beyond d, and repeat to include the last three dots. This accomplishes connecting all the nine dots with three lines (and two extra lines) without lifting the pen from paper.
sticky...
re(2): more difficult
