Everybody loves a good April Fool’s Day gag. Okay, not everybody. Most people love a good April Fool’s Day gag, and these days it seems like April Fool’s Day and the internet are like peanut butter and chocolate: they weren’t made to go together, but they make for a really sweet combination. There is, of course, plenty of room for debate about what the best April Fools Day gag is – YouTube on DVD? Conan O’Brien taking over Twitter? An e-lane for texting pedestrians?

One joke in particular stands out, if not for its humor, than for sheer elaborateness. You may or may not be familiar with xkcd, the self-described “webcomic of romance, sarcasm, math, and language.” If not, you owe it to yourself to check it out – an evening or two working your way through the backlog is well worth it, too. At any rate, xkcd author and former NASA engineer Randal Munroe used the site for a remarkably complex April Fool’s Day gag yesterday. Visitors to the xkcd site found that they were seeing different comics based on their location. For example, people in Toronto saw the following:

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Those in Michigan saw something similar, while those in North Carolina (and Florida) saw a variation that replaced snow with hurricanes:

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But that’s not all. It turns out that there were even more variations. This one showed up in France:

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Impressive, right? Well, it turns out there’s more. Upon further investigation, users on reddit and the xkcd forum found that which comic you were seeing varied by a whole lot more than just where you live. They found variations based on referrer, browser (see the lead image above for one of the three Google Chrome variants), ISP, and more.

Here’s what people who got to the site from reddit saw:

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Visit from an AT&T device, and there’s a good chance you saw this:

There were even some variations based on your device. One reddit user visited the site from a Kindle e-reader (the e-ink variety, not a Kindle Fire), and saw the following:

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Suppose you tried to cheat. Maybe you thought that by putting your browser into incognito mode you could fool the system. Well, apparently not:

With all the effort taken to make different comics appear to different people, the system didn’t always work perfectly. A few users saw Friday’s comic. There also appears to have been a more generic comic shown to many who navigated to the site directly:

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By now you may be asking yourself, “What’s the point?” Sure it’s funny to give everybody a different comic. This is xkcd, though, and things often go a bit deeper than that. If you’re familiar with xkcd at all, you know that often the real joke of a comic is in the alt text (the text that appears if you hover over a comic with your mouse. Well, the same goes here, as well. No matter what comic you may have seen yesterday (and today), they all have the same title and the same alt text. The title is always “Umwelt,” and the alt text contains a definition:

Umwelt is the idea that because their senses pick up on different things, different animals in the same ecosystem actually live in very different worlds. Everything about you shapes the world you inhabit –from your ideology to your glasses prescription to your browser window size.

If you’ve ever had the thought that, say, a bug must see a completely different world than you do because of its perspective, that’s what Umwelt is. It looks like Munroe decided to give xkcd’s fans a little dose of Umwelt on a large scale yesterday by making every fan’s experience just a little different. While it may not be the funniest April Fools day gag around, it’s got to be one of the cleverest.

Are you an xkcd fan? What do you think of the Umwelt gag? Is it funny, or just weird? Which comic did you see? Let us know in the comments.