The police had discovered two spies, which were both living in the same hotel. Instead of arresting them straight away, the police decided to intercept their messages, and see what they were planning. The police had two detectives follow the men around. Much to the detectives amazement, the two spies never met or talked to each other. They realized that the only place the men could communicate through was the hotel's bathroom.
The police then searched the bathroom from top to bottom, they searched in and around everything in the bathroom. They had to finally give up, concluding that the spies did not communicate through the bathroom. They were about to leave, when one policeman came up with the answer.
Where, out in the open, could the spies leave their messages, and still not have anyone see them?
HINT: What could they have done in the bathroom that might have given them writing material?