if you ever meet a perfect version of yourself constructed by aliens from radio transmissions, don't shake their hand.
For those who don't know, this refers to a story in the Feynman Lectures on Physics. Here's my version; I've taken some liberties.
Imagine you're on the phone with an alien who speaks English, except they don't know what "left" and "right" mean. You want to explain it to them so they know which tentacle they should use to shake the right-handed President's hand if they should ever meet. The alien can be anywhere in the universe, so you can't refer to stellar positions or similar, leading you to devise an experiment for them to perform.
Your initial attempts use gravity, electricity, and magnets, but you notice each experiment comes out essentially the same if you swap "right" and "left"--for instance, you could give the alien instructions for making a clock in hopes of defining "right" using clockwise rotation, except if the alien made the clock exactly backwards by reversing the notion of "left" and "right", they wouldn't be able to tell. A particle physicist happens by and tells you about a magical experiment involving the weak nuclear force that *does* distinguish left and right inasmuch as the experiment fails if the alien screws up "right" and "left" and succeeds otherwise. (For the curious, some more details here [lbl.gov] and here [wikipedia.org].) Great, problem solved.
"But wait!" the physicist says. "The alien needs to use regular matter instead of antimatter in the experiment. The results will be reversed otherwise! Come to think of it, I have no idea how to tell them the difference between matter and antimatter. If you ever meet them and they start trying to shake your left hand, RUN, since the alien will be made of antimatter!"
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