Fill up the 1/2 cup entirely. Then pour the contents of that into the 1/3 cup until that is full. Make sure not to spill. The leftover amount in the first cup represents 1/6 because of the work shown below.
1/2 - 1/3 = 3/6 - 2/6
1/2 - 1/3 = (3-2)/6
1/2 - 1/3 = 1/6