In the simplest case, involving only two people, the couple begins by walking down a long hallway from darkness into sunlight. (The hallway is wide enough for them to walk side by side, but too narrow to accommodate more than two people at a time.) At the end of the hallway, the couple will find two sets of footprints, on which they'll stand facing one another, nearly touching. (Depending on their preference, they may be dressed or naked.) They'll look up and see a window bright with sunlight, and suspended in that window above their heads they'll find the entanglement apparatus, which has been precisely calibrated (using a system of adjustable prisms) to divide the sunlight passing through a nonlinear crystal (made of beta-barium borate) so that half of the light shines on each person's face. They'll stand for approximately a minute, allowing countless entangled photons to bombard their skin, gently entangling their flesh by the photoelectric effect. Then they'll turn away and walk back down the hall and out into the open.