An engineer, physicist and topologist are sent to three identical rooms
and presented with the same problem - they have to open a can without a
can opener.

The scientist in charge of the experiment gives them half an hour, and
then goes to check on them.

He visits the engineer first - when he walks in, he see the engineer
lying, exasted on the floor, the walls dented in several places and the
can split and scattered on the floor. He's opened the can by repeatedly
throwing it against the wall. Happy enough with the result, the scientist
moves on to the physicist.

When the scientist entered the physicist's room, he saw that he had made a
number of calculations on the wall with a marker, and that there was a
single, carefuly calculated dent in the wall, with the can open on the floor
beneath it. The scientist ticked off the room and moved onto the topologist.

However, when he entered the third room, the topoligist was nowhere to be
seen - the walls were covered with calculations, but the can was sitting
unopened in the middle of the room. The scientist looked around for a
while, and was about to leave until he heard the can shake about - so he
quickly opened it with his can opener, and the topologist climbed out.

"Thank goodness", the topologist exclaimed, as he gasped for air. "I got a
sign wrong!"