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If you want to know if there is any mail inside your mailbox, you merely need to walk out and take a look. From that one, simple observation you can deduce the postal service has been and popped a belated birthday card from Aunt Judy in through the slot. Thanks Aunt Judy!
This intuitive system can be described as local realism. You can imagine a chain of events from Aunt Judy to yourself taking place, each moving through a series of connected locations from Aunt Judy's kitchen table, to the post office, to your address.
For most things in science, local realism does a good enough job of describing how things are.
But then scientists had to come along and mess it all up with quantum physics, discovering the machine running reality operates by rules that make little sense. Arguably the most bizarre of which contradict local realism.
Accordingly, before you look inside the mailbox, the Universe has yet to settle on whether a card is inside, if a postal service has been by, or if Aunt Judy has remembered your birthday.
Physicists refer to these undecided objects and events as contexts, and their relationship to one another as entanglement. The only way to know if Aunt Judy sent a card is to look in the mailbox and check – before then, it was all a big shrug as far as reality goes.
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