During a class of calculus my lecturer suddenly checked himself and stared intently at the table in front of him for a while.

During a class of calculus my lecturer suddenly checked himself and
stared intently at the table in front of him for a while. Then he
looked up at us and explained that he thought he had brought six piles
of papers with him, but "no matter how he counted" there was only five
on the table. Then he became silent for a while again and then told
the following story:

"When I was young in Poland I met the great mathematician Waclaw
Sierpinski. He was old already then and rather absent-minded. Once he
had to move to a new place for some reason. His wife wife didn't trust
him very much, so when they stood down on the street with all their
things, she said:
- Now, you stand here and watch our ten trunks, while I go and get a
taxi.

She left and left him there, eyes somewhat glazed and humming
absently. Some minutes later she returned, presumably having called
for a taxi. Says Mr. Sierpinski (possibly with a glint in his eye):
- I thought you said there were ten trunks, but I've only counted to nine.
- No, they're TEN!
- No, count them: 0, 1, 2, ..."