"See, how they work," the first said, "is the right lens is polarized vertically and--"

"Wait, what do you mean by 'polarized'?" said the second.

"A vertically polarized lets no horizontally polarized light in, and a horizontally polarized lens lets no vertically polarized light in," said the third. "If two perpendicularly polarized lenses are put together, no light is let through."

"So as I was saying, the right lens is polarized vertically, and the left horizontally, or vice versa, such that each eye sees a different picture."

The second had an idea. "So that means if I..." He closed his right eye, and smiled. "Yes, I can only see your right eye."

The fourth and final person (you, of course), sensed something wrong. First, you verified the second's observation. Then, you said...