Fun puzzle - connect boxes

[mouser]

From http://boingboing.net/2017/01/05/good-puzzle-connect-the-six-b.html

[attach]

[tomos]

Clever

[mouser]

Btw if anyone is struggling, this isn't a trick/joke, there really is a solution.  Here's a clue for people who get stuck:

Spoiler (click to show/hide)

Print out or draw the picture, then connect 1 with 1, and 2 with 2, now see if you can figure out how to connect 3 to 3.

[anandcoral]

Very good indeed.

Spoiler (click to show/hide)

One shd try on paper instead of screen to understand the solution.