If you're only doing 1 at a time, I apologize, but I have one for the rest of use while Bossk is working on rics.
You have 9 dots, arranged in a 3x3 grid, looking something like this (draw one out for yourself):
. . .
. . .
. . .
Now, using only 4 straight lines and without picking up pencil, put a line through all 9 dots.