5: Blown apart
Title text: Blown into prime factors
This comic is a mathematical and technical joke involving prime numbers and primary colors.
In the comic, an anthropomorphic black-colored number 70 sees a package, but it turns out to be a letter bomb that explodes when opened. The result is pieces of the number scattered about. The specific pieces are a red-colored 7, a green-colored 5, and a blue-colored 2.
The title text explains the logic for splitting 70 into 7, 5, and 2; as with many of the earlier comics, the title text explains the joke rather than adding to it. 7*5*2 is a prime factorization of the number 70. Prime numbers are numbers that cannot be divided by any number other than itself and 1. Factors of a number are numbers that can be multiplied together to produce that number (i.e. 2×5×7 = 70). 70 has other factors, including 1, 10, 14, 35, and 70, but 2, 5, and 7 are the only factors that are prime. All other factors of 70 can be formed by choosing zero, two, or three of the prime factors and multiplying them together.
An implication of this comic is that prime numbers would be immune to explosions, as they are already their smallest parts.
Although not explicitly called out, the colors of the numbers also seem to have been blown apart. Red, green, and blue are the primary colors in the additive color model. These colors mixed in pairs produce cyan, magenta, and yellow, which are primary colors in the subtractive color model. The removal of all additive primary colors, or conversely, the combination of all subtractive primary colors, produces black, which is the color of the original 70 (according to the official transcript, although it looks dark blue in the drawing). The comic is somewhat misleading in that red, green, and blue do not compose black in either color model, but the difference between the two models is not widely understood (most still view the additive primaries as red, yellow, and blue).
- [A black number 70 sees a red package. This small panel is partly overlaid on the next larger panel, which is shifted down.]
- 70: hey, a package!
- [The package explodes in a cloud of brown smoke. This panel is both behind the first in the top left corner, and below the last panel, which has been laid on top of that corner.]
- [There are a red 7, a green 5, and a blue 2 lying near a scorched mark on the floor.]
- This comic was posted on xkcd when the web site opened on Sunday the 1st of January 2006.
- It was posted along with the 41 comics posted before that on LiveJournal.
- On the site, it is the first comic with an actual punchline (all previous comics are either just sketches or establishing the background for later comics)
- But this comic, as well as 12: Poisson also released that day, were never posted on LiveJournal.
- The release date is given from - wayback.archive. At least the 25th of October was the first crawl date.
- This also explains why it was "released" on a Tuesday.
- On xkcd, it was released for the first time to the public on Sunday the 1st of January 2006.
- One of the original drawings drawn on checkered paper.
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