Beautiful results in TCS Recently, a friend of mine (working in TCS) mentioned in a conversation that "he wanted to see/know all (or as much as possible) of the beautiful results in TCS in his lifetime". This kind of made me wonder about the beautiful results in this area and hence the motivation for the following question:

Which results (or ideas), in your opinion, are beautiful in theoretical computer science? It would be great if you mentioned the reason as well. 
[Would also be fine even if the ideas originate in mathematics, but have aroused interest and found uses in TCS]

I would start with an answer as Cantor's diagonal argument because it's simple, elegant and yet a powerful result.
The undecidability of the halting problem. 

Beautiful for many reasons. It is an impossibility result. The proof uses diagonalisation. The statement applies to a broad range of models of computation. It can be formulated in a variety of ways, particularly, using standard programming languages. It was a watershed result in the history of computing. Extending this statement leads to Rice's Theorem, Turing degrees and many other cool results. Etc. Etc. Etc.