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.
go from point g(1,1) to (4,1),then draw a line from (4,1) to (1,4)passing through b(3,2) and f(2,3).then draw the line from (1,4) to g(1,1).now draw the line from g(1,1) to c(3,3).