Headache
Headache
[Megan has a bike, and is wearing a helmet. Cueball is at a computer.] Megan: Wanna go for a bike ride? Cueball: Nah, I hate 3D stuff. It gives me a headache. When you think about it, this excuse can get you out of almost anything.

This was the second April fools' comic released by Randall , but in principle the first real one to be released on April 1st, which in 2011 fell on a Friday, a normal release day for xkcd. The previous fools comic was literally Not Found . After this comic Randall began releasing April Fools comic every year on April 1st disregarding the weekday of that date. The next was 1037: Umwelt released on Sunday April 1st 2012, the first to use another day of the week than Monday, Wednesday or Friday.
Some people suffer from headaches, eyestrain, motion sickness and other problems when watching 3D movies , playing 3D games, watching 3D television , playing hand-held Nintendo 3DS , etc.
In this comic, Cueball is using the excuse that 3D gives him a headache to get out of going outside into the real world, where everything is in 3D . Instead he stays inside and looks at his 2D computer monitor. In the title text, he says he will only go to flat places (i.e. places where everything he could see would be 2D-like).
This comic was released on April Fools' Day . The April fools joke for 2011 made every comic on the site 3D, thus forcing people like Cueball to endure 3D even at their computer screens. An exception to this is 848: 3D , for which the third dimension is not visible. The 3D view is still available at xk3d.xkcd.com for all comics prior to this one.
In his Øredev 2013 talk Randall mentions that a few of his friends created this 3D view and told him just 1.5 hours before the scheduled comic for that day would go live. Randall quickly drew a 3D themed comic to match.
The title text suggests that Cueball is only comfortable seeing objects with visible flat surfaces like flat movie and TV screens (which the entertainment industry would not call "3D").
[Megan has a bike, and is wearing a helmet. Cueball is at a computer.] Megan: Wanna go for a bike ride? Cueball: Nah, I hate 3D stuff. It gives me a headache. When you think about it, this excuse can get you out of almost anything.
