For an easier to understand analogy, you and your roommate are going different vacations, and you both suck at cleaning your apartment. Both of have your shoes in a big pile. You grab two shoes without looking and stuff them in your suitcase on your way to the airport. Likewise, your roomie, packing at the last minute just like you, grabs the other two shoes without looking, packs them, and heads to the airport. You go to Hawaii, they go to London. When you arrive at your hotel, you unpack, and find that the left shoe is yours, but you have your roommate's right shoe. You know that when your room-mate lands in london, and they observe their luggage, they will find that they have their own left shoe and your right shoe.

by reddit user LiamtheV