An old chestnut goes as follows:

There are three houses and three utilities, like so:

      H1      H2      H3

      U1      U2      U3

How can you connect each house to each utility, by drawing lines without crossing? (Pretend this is on a sheet of paper; you can't cross around to the back of the paper, make holes in the paper, or run utility lines through the houses or other utilities.