|Date||Jul. 12th 2010|
Dilution is the 765th xkcd comic.
Homeopathy is a form of alternative medicine which uses highly diluted substances as medicines. Such substances are often diluted (with water) so highly that, on average, less than one molecule of the substance is left in the "medicine". Homeopaths believe that (to some extent) higher dilutions are more effective than the pure substance, with less risk of side effects.
A dilution of "30X" means that the substance is diluted by a factor of ten 30 times; it would be present in 10<sup>-30</sup> (or 0.000 000 000 000 000 000 000 000 000 001) of its starting concentration. Since a typical ejaculation contains only about 10<sup>9</sup> or 10<sup>10</sup> sperm cells, a 30X dilution of semen (like in the comic) would be unlikely to contain even one sperm cell, and would be even less likely to cause a pregnancy. Any genes that made a person reproduce this way would probably not be passed on (since that someone would probably have no children), and would tend to evolve out (i.e., go exinct).
"Dear editors of Homeopathy Monthly: I have two small corrections for your July issue. One, it's spelled "echinacea", and two, homeopathic medicines are no better than placebos and your entire magazine is a sham."
Many magazines and journals will publish corrections with their "letters to the editor" section. The title text is a letter to the editors of the (fictional) magazine Homeopathic Monthly. However, it's unlikely that said editors would publish this letter, at least not in full.
Controlled scientific trials have consistently shown that homeopathic medicines are no more effective than placebos. This isn't very surprising, since, given their high dilution (see above), they pretty much are placebos. Some homeopaths presumably know this, but are still making money off people who don't (or who won't believe it).