How do you connect 9 dots only using 4 straight lines and not going over the same line twice?

To facilitate the description of this solution, let us give each of the 9 dots arranged in a 3 by 3 grid a number from 1 to 9:

1 2 3

4 5 6

7 8 9

Draw the first line beginning at dot 1 and continuing diagonally down and right, pass through dot 5 to reach dot 9.

From dot 9, the second line is drawn upwards, passing through dot 6 and dot 3 to end above dot 3.

The key to solving this puzzle with 4 straight lines joined continuously -- often described as "drawn without lifting the pencil off the paper" -- is to end the second line far enough above dot 3 that the third line drawn diagonally down and left will pass through dots 2 and 4, ending left of and level with dot 7.

Finally, the fourth and last line is drawn horizontally across to the right, passing through dots 7 and 8, ending back at dot 9.

Image for visual aid:

See the related link, "Image for visual aid", below.