2 puzzles from Frank P. Ramsey, each explained in a minute and credited to the original work. Free on Savvy.
At any party of six people, prove that there must be either three people who all know each other, or three who are all strangers to one another. Why does a party of only five peopl…
— Frank P. Ramsey, On a Problem of Formal Logic
At a party of six people, any two either know each other or are strangers. Prove that there must always be at least three people who all know each other, or three people who are al…
— Frank P. Ramsey (underlying theorem), Theorem on friends and strangers