A library has two rooms, one for general books and one for restricted books. The restricted books are not allowed out of the library, and no notes or recordings are allowed to be taken out of the restricted room.
An attacker wants to sneak information out of the restricted room. To do this the pick up a pile of non-restricted books and go into the restricted room. Depending on what they read in there they rearrange the pile of non-restricted books into a particular order. A guard comes along and sees them, they are thrown out of the restricted room and their pile of non-restricted books is put on the issue desk ready to be put back into circulation.
Their conspirator looks at the order of the books on the issue desk and decodes a piece of information about the book in the restricted room. They repeat this process about 500000 times a second until they have transcribed the secret book.