Physicists seem to have a reputation, self-made or otherwise, for discussing physics on writing surfaces that are . . . less than typical. The most common phrase for a quick, short, but somewhat accurate calculation is a "back-of-the-envelope calculation," implying the nature of the writing surface: whatever is at hand.
In the CERN cafeteria, over mouthfuls of paella and sips of water, sometimes your discussion about contributions to electrically charged particle radiation demands a writing surface. The napkins were all used, no envelopes in sight, and suddenly my colleague (an SMU post-doctoral researcher named Aidan) said, "We can use this banana peel - it works quite well!" He began scribbling a diagram of radiation loss versus energy for a few processes, writing in swift, smooth strokes on the waxy but rough surface of the banana peel. Believe it or not, it does make a FABULOUS writing surface!