A 'domino tiling' of a region of the plane is a way of covering it (and only it) completely by non-overlapping dominoes.
For example: There's 1 domino tiling of a 2 by1 rectangle & 2 tilings of a 2 by 2 rectangle.
(1 consisting of 2 horizontal dominoes & 1 containing of 2 vertical ones)
How many domino tilings of a 2 by 10 rectangle are there?