"…You can always tell the particles apart, in principle—just paint one of them red and the other one blue, or stamp identification numbers on them, or hire private detectives to follow them around. But in quantum mechanics the situation is fundamentally different: You can’t paint an electron red, or pin a label on it, and a detective’s observation will inevitably and unpredictably alter its state, raising doubts as to whether the two had perhaps switched places”

—David J. Griffiths Intro to Quantum Mechanics 2nd Ed. page 204