[pdpi]

The punchline is that you can fill in the centre of Escher's piece by using complex analysis, and it produces a very satisfying, "obviously correct", solution.

But, as with all jokes, the punchline isn't funny at all without the setup.

[ant6n]

The joke is that if you fill in the center, it shows the Droste effect of the image and kind of diminishes the magic of it.