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* '''ρ (lowercase rho): Unfortunately, the test vehicle suffered an unexpected wing separation event.''' — Used in statistics to measure the association between variables. Lowercase rho often represents volumetric mass density, such as the density of air that a wing might be traveling through. The density of a fluid is directly proportional to the Reynold's number, which dictates the sort of physics used to model motion through the fluid. Flying a plane in conditions with a Reynold's number well outside of what it was designed for could have catastrophic consequences. A variant symbol ϱ with the same meaning is common in European handwriting.
 
* '''ρ (lowercase rho): Unfortunately, the test vehicle suffered an unexpected wing separation event.''' — Used in statistics to measure the association between variables. Lowercase rho often represents volumetric mass density, such as the density of air that a wing might be traveling through. The density of a fluid is directly proportional to the Reynold's number, which dictates the sort of physics used to model motion through the fluid. Flying a plane in conditions with a Reynold's number well outside of what it was designed for could have catastrophic consequences. A variant symbol ϱ with the same meaning is common in European handwriting.
  
* '''Ξ (capital xi): Greetings! We hope to learn a great deal by exchanging knowledge with your Earth mathematicians.''' — Probably the least used Greek letter in math and physics despite being easy to write and recognize. According to the comic, anyone using this letter is likely a being from another planet. It does see very occasional use, such as in the Riemann xi function or as the symbol for a class of heavy baryons in particle physics. It resembles but is not to be confused with a "hamburger button" or a triple equals sign ≡. Coincidentally, it also resembles the Chinese or Japanese character for the [https://en.wikibooks.org/wiki/Chinese_(Mandarin)/Numbers number 3] (三). Randall thinks it most closely resembles alien writing.
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* '''Ξ (capital xi): Greetings! We hope to learn a great deal by exchanging knowledge with your Earth mathematicians.''' — Probably the least used Greek letter in math and physics despite being easy to write and recognize. According to the comic, anyone using this letter is likely a being from another planet. It does see very occasional use, such as in the Riemann xi function or as the symbol for a class of heavy baryons in particle physics. It resembles but is not to be confused with a "hamburger button" or a triple equals sign ≡. Coincidentally, it also resembles the Chinese or Japanese character for the [https://en.wikibooks.org/wiki/Chinese_(Mandarin)/Numbers number 3] (三). Randall thinks it most closely resembles alien writing.
  
 
* '''ψ (lowercase psi): You have entered the domain of King Triton, ruler of the waves.''' — Both capital and lowercase psi are shaped like tridents. In classical mythology, {{w|Triton}} is one of the gods of the sea, alongside his father Poseidon, and tridents are commonly associated with sea gods. In quantum mechanics, either psi is used to represent the wave function of a particle, leading to a pun. (Psi is also used in mathematics to represent the sum of the inverse of the Fibonacci numbers, the division polynomials, the supergolden ratio, and other purposes.)
 
* '''ψ (lowercase psi): You have entered the domain of King Triton, ruler of the waves.''' — Both capital and lowercase psi are shaped like tridents. In classical mythology, {{w|Triton}} is one of the gods of the sea, alongside his father Poseidon, and tridents are commonly associated with sea gods. In quantum mechanics, either psi is used to represent the wave function of a particle, leading to a pun. (Psi is also used in mathematics to represent the sum of the inverse of the Fibonacci numbers, the division polynomials, the supergolden ratio, and other purposes.)

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