How many ways are there to tile a 2 x n grid with 1 x 2 dominos?
This problem can be solved using Fibonacci numbers. Let Sn be the number of valid tilings of the 2 x n grid. Each such tiling has either a vertical 1 x 2 domino or two horizontal dominos on the right. Therefore, each tiling of a 2 x (n-1) grid or a 2 x (n-2) grid generates a tiling of the 2 x n grid, and hence we have a recurrence relation Sn = Sn-1 + Sn-2. This is precisely the recurrence relation of the Fibonacci numbers. Checking our base cases, we see that there is one way to tile a 1 x 2 grid and two ways to tile a 2 x 2 grid, so S1 = 1 and S2 = 2. Therefore, the number of tilings is precisely the Fibonacci sequence.