One was a droll, sarcastic satirist; the other was a celibate mad scientist. Both of them were brilliant and probably a little bit crazy, but in completely different ways. That's why Mark Twain and Nikola Tesla have to rank right at the top of the "We'd love to hear what they talked about when they were alone" list.
Twain and Tesla's friendship was forged more or less from geeking out as fanboys of one another. Tesla had read Mark Twain before coming to New York City to invent the 20th century, and the only thing Twain knew of Tesla was his AC polyphase system. As Twain wrote, "I have just seen the drawings and description of an electrical machine lately patented by a Mr. Tesla, and sold to the Westinghouse Company, which will revolutionize the whole electric business of the world. It is the most valuable patent since the telephone."
So when the two met at parties around New York City, the relationship developed like a childhood friendship where one kid has a bunch of cool toys and the other knows a bunch of great jokes. Twain would even visit Tesla's workshop and offer himself up as a guinea pig for Tesla's new inventions.
During one such playdate, Tesla revealed his mechanical oscillator that could produce alternating currents. One of the side effects was apparently some pretty substantial vibrations, which Tesla suspected might be therapeutic. Sure enough, this was all Twain needed to hear, and he immediately volunteered to be a test subject.
Twain leaped into the lap of the machine and told Tesla to zap his brains out. After a few minutes of insisting that he felt like the machine was giving him, "vigor and vitality," Twain quickly realized what he was actually feeling was the machine literally shaking the shit out of him. He had to rush to the restroom, thus proving Tesla had discovered the first and only electric laxative.
If there was ever a duo that could have wiped out war if given enough electricity and shit-machines, this was it.