Beautiful theorems with short proof I like to ask for beautiful mathematical theorems with short proof. A proof is short in my sense if it is at most one page assuming basic notations and very basic results a second year student will know and understand. I do not like to discuss what beauty means...I have now written down a script with 107 such theorems (in my sense) and their proof and I have ideas for another 20. One the one hand I would be very thankful for theorems I have yet not considered. On the other hand I like to see if there is some consensus which theorems with a short proof are beautiful.
 A: Picard's little theorem is remarkable, and its one-line proof led Littlewood to remark that it would be the world's shortest Ph.D. thesis.  
A: L.M. Kelly's proof of the Sylvester-Gallai theorem: in any configuration of $n$ points in the plane, not all on a line, there is a line containing exactly two of the points.
See Aigner & Ziegler, "Proofs from the Book", chapter 8.
A: Fermat's proof, by infinite descent, that there is no Pythagorean right triangle whose area is a square might qualify.
A: Euler's formula $$\mathrm{e}^{i \theta} = \cos \left( \theta \right) + i \sin \left( \theta
\right)$$ when considered as a theorem. From whatever angle you look at it, almost all the proofs are short and extremely beautiful.
A: The proof(via the pigeon-hole principle--continued fractions would need too much preparation) that when D>0 is not a square then the "Pellian equation" xx-Dyy=1 has a non-trivial solution.
A: The standard evaluation of $\int_{-\infty}^{\infty} \exp(-x^2) dx.$
A: Cantor's diagonal argument to prove that $\mathbb{R}$ is uncountable.
A: @Paul Monsky's proof of Monsky's theorem: a complete proof starting from nothing takes two pages. (doesn't quite meet the criteria, but what the heck).
