Solve the riddle: I am a polygon. I have two right angles. I have only one pair of parallel sides. What am I?
A trapezoid, or any number of other polygons.
A trapezoid is the polygon with the fewest sides fitting the criteria, however there are multiple polygons which suffice (since it was not specified that the polygon must be regular).
A few examples should show how to generate a polygon with any number of sides what has two right angles and a single pair of parallel sides.