This one can better be told in a pub. First three points on the table: a b c On a lies a beermat and on c stands a glass. The mathematican has to move the c to a. He takes the glas and puts it on the beermat. Now the glas is put on point b and the mathematican has to move it to a. The mathematican takes the glas and puts it on c – the problem has been reduced to one already solved.