Main Page
Welcome to the explain xkcd wiki!
We have an explanation for all 3222 xkcd comics, and only 70 (2.2%) are incomplete. Help us finish them!
Latest comic
| Dental Formulas |
 Title text: I mean, half of these are undefined. And your multiplication dots are too low; they look like decimal points. |
Explanation
A dental formula specifies the typical number and location of teeth of each type for a given species. There are two rows, representing the upper and lower jaw, separated by a horizontal line. On each row, the number of each type of tooth is given for one side of the jaw, with dots separating the numbers. The number of incisors is indicated first, canines second, premolars third, and finally molars. The formula in the comic would represent 3 incisors, 1 canine, 3 premolars, and 1 molar on each side of the upper jaw, and equal numbers in the lower jaw, with the exception that there are only 2 premolars; this is the dental formula for the cat family. The adult human dental formula is 2.1.2.3 for both the upper and lower jaw.
Cueball is mistakenly treating a dental formula as an arithmetic expression, with the line indicating division and the dots indicating multiplication: 3⋅1⋅3⋅1 divided by 3⋅1⋅2⋅1, giving 9/6 = 3/2. Since the numbers involved are always small natural numbers, calculating the results when treating them this way would be fairly trivial, which is why he is surprised at the effort given to studying them.
In the title text, his statement that half the formulae are undefined refers to animals that lack one of the four types of teeth in the lower jaw, leading to a zero in the lower part of the dental formula. Cueball is attempting to multiply all terms in that lower part, giving a result of zero, and then treat that as a mathematical denominator, resulting in an undefined division expression. He also notes that the "dots are too low", as the dots in a dental formula are period characters, whereas multiplication in mathematical formulae uses middle dot characters (except in the Commonwealth, where this is sometimes reversed).
The word 'mammologist' is an alternate spelling of 'mammalogist', meaning one who studies mammals (or, in some cases, specifically studying the mammaries (i.e. breasts) which mark out mammals in general). Unlike odontology (dentistry, not to be confused with odonatology), which studies the health of a patient's teeth, mammalogy studies teeth as a means to identify species and what they eat.
Transcript
- [Cueball and Megan are standing in front of a whiteboard, on which is written
- 3.1.3.1
3.1.2.1
- 3.1.3.1
- along with a drawing of a tooth and some other scribbles.]
- Cueball: Do mammologists think these are hard?
- Cueball: I mean this one just evaluates to 3/2.
- [Caption below the panel:]
- Mathematicians encounter dental formulas
View comic discussionNew here?
Last 7 days (Top 10) |
|||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
You can read a brief introduction about this wiki at explain xkcd. Feel free to create an account and contribute to the wiki! We need explanations for xkcd comics, characters, What If? articles, and everything in between. If it is referenced in an xkcd comic, it should be here.
- If you're new to wiki editing, see the explain xkcd:Editor FAQ for a specific guidance to this wiki and the more general help on how to edit wiki pages. There's also a handy wikicode cheatsheet.
- Discussion about the wiki itself happens at the Community portal.
- You can browse the comics from the list of all comics or by navigating the category tree at Category:Comics.
- The incomplete explanations are listed here. Feel free to help out by expanding them!
Rules
Don't be a jerk!
There are a lot of comics that don't have set-in-stone explanations; feel free to put multiple interpretations in the wiki page for each comic.
If you want to talk about a specific comic, use its discussion page.
Please only submit material directly related to xkcd and, of course, only submit material that can legally be posted and freely edited. Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.
If you need assistance from an admin, post a message to the Admin requests board.
