Still in awe of this incredibly clear visual explanation of how/why Diffie-Hellman key exchange works.
Two parties want to agree on a shared secret, but they don't want a passive observer to be able to learn about it.
https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
https://commons.m.wikimedia.org/wiki/File:Diffie-Hellman_Key_Exchange.jpg