A two-part lecture course on Topology, by Professor N. The first part is in the spring semester, the second part in the following autumn semester; between them is the long summer break.
First week of the autumn semester, first lecture. Professor N comes into the room. He walks to the blackboard and picks up the chalk. He turns to the class and starts:
"Hence, ... "
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[Anectdote told by Paul Halmos in his autobiography. I quote from memory, so the details will vary.]