In 1935, Einstein wrote a paper with Boris Podolsky and Nathen Rosen trying to expose the weird behavior of quantum mechanics calling it “spooky action at a distance”. Among its many weird behaviors, the notion of quantum superposition really defies our imagination.
Even weirder, if one looks into a quantum system of say two electrons and they are in an entangled state, if you measure the property of one electron, say its rotation, you can tell what the other electron’s rotation is — without even bothering to measure it. Is it weird or astonishing? I think both.