1 Haken: {again walking, at the head of the group} How are we going to get out of this verdammte labyrinth, Erwin?
2 Erwin: A mathematics professor at Darmstadt once told me a foolproof way to navigate out of a maze, Herr Kolonel.
3 Erwin: All you have to do is always keep your left hand on a wall as you walk.
4 {Haken holds up the hook that replaces his left hand}

This simple rule for solving a maze works for simply connected mazes, i.e. ones in which there are no disjoint wall sections. If this is not the case, then the rule might not work.

I debated a lot of possibilities for dialogue in the final panel. But in the end I decided the emphasis really needed to be on the visual to make the gag work as I wanted, and any dialogue at all would just distract from that.

But, so you can't complain, here are the possibilities I discarded:

Erwin: Oh...

Haken: Ahem.

Haken: Ahem.
Erwin: Or someone's left hand, I suppose.

Erwin: Or someone's left hand, I suppose.

Haken: Did your professor have one of these?
Erwin: Ah. Nein.

Haken: Never the left! Follow the right wall.
Erwin: ... that's how I failed that class.

Haken: Erwin. Remind me to visit this professor if we ever get out of here.
Erwin: Er... jawohl.

I go through this process quite a bit during post-production of the strips, when I'm actually adding the captions, despite having a typed up script for the strip that I follow when I'm arranging the photos. In this case, my original script left the last panel empty of dialogue, but other possibilities occurred to me as I assembled it.

One change I did make to this script at post-production was to change Erwin's last line from "always keep your left hand on a wall as you walk" to "always keep your left hand touching a wall as you walk". I figured "touching" would have a stronger impact than simply "on" when Haken presents his hook, since you can have a hook on something, but you can't use your sense of touch through the hook.

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2014-04-28 Rerun commentary: Erwin's method, as well as other methods which work on more complex mazes, are described and discussed in Wikipedia.

Erwin's method also assumes that at no point do you ever run into a hungry Minotaur, so it definitely has its limitations.