"At Vladimir [prison] we had been mixed together with the criminals; at Christopol we never saw them. But among ourselves there was intense intercell communicaiton. The toilets were the most effective means, but it took a lot of work to remove the water, and the toilets were near the door, where the guards could see you. We also communicated through the radiators. You would press your mug against the heating pipe and speak into it; to listen, you'd turn your mug upside down. A third method was tapping to the next cell in Morse code, and a fourth involved throwing a note into the next exercise yard."
From: Fear No Evil, by Natan Scharansky, Random House, New York, 1988