Sometime between breakfast and lunch each day, the guard brought a hammer and chisel to the prisoner. The prisoner chipped another straight line in the concrete wall, then passed the tools back to the watching guard.
One day the prisoner turned to carve the day’s line, but an idea stopped him. After clattering the tools together, he returned them, leaving no new scar on the wall. For two weeks he repeated this, and at the end he looked at the empty patch of wall. No marks left meant no days had passed. He had created limbo in the eight-by-twelve cell.