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Explain xkcd: It's 'cause you're dumb.
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Geese
Anyway, that's a common misconception. Geese live for a long time; all the ones we can see will probably keep flying around for billions of years before they explode.
Title text: Anyway, that's a common misconception. Geese live for a long time; all the ones we can see will probably keep flying around for billions of years before they explode.

Explanation

Megan is commenting on a flock of geese passing overhead and says the light from the geese reaching their eyes now could have come from hundreds of years ago. This is a fact for the light from stars. Cueball points out the absurdity of Megan's statement by pointing out that the geese are only a few hundred yards away rather than a few hundred light years. She continues along the same lines when she implies to Cueball that he is observing a past version of her, despite them being only a few feet apart. (Technically he is viewing a past version of her, but not one from "long ago"; if someone is two feet away from you, you are seeing them as they were roughly 2 nanoseconds ago.)

In the title-text Megan continues to treat the geese as if they were stars, which "live" for a few billion years before exploding. Most stars visible with naked eye are within a thousand light-years of Earth, (as discussed in 1342: Ancient Stars), and it's unlikely that any star Megan currently sees actually exploded within the relatively short span of last few thousand years.

Megan's statement "You're hearing how they once sounded." is somewhat more justified -- sound from "a few hundred yards away" would take about one second to be heard (depending on the exact distance and the prevailing atmospheric conditions (which affect the speed of sound)). That said, the sound of a goose isn't likely to change enough over the course of a second or two to make this distinction particularly significant.

The strip may also take inspiration from Gamow's "Mr Tompkins" stories which were designed to help laymen understand some of the consequences of relativity and quantum mechanics. In one of the stories Mr Tompkins visits a town where the speed of light is 30 miles per hour. For the light to have taken hundreds of years to go from the geese to Megan and Cueball, the speed of light in this strip would have to be much slower than in Gamow's story.

Transcript

[Geese fly in V-formation. Megan and Cueball are lying on the ground, watching them.]
Megan: To think... we're seeing light that left those geese centuries ago.
Megan: By now, they could be long dead.
Cueball: ... What? They're a few hundred yards away. I hear them honking.
Megan: Ah, yes. You're hearing how they once sounded.
Cueball: You're very weird.
Megan: Or I was, long ago...

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