Is a Trapezoid a Parallelogram?
Quadrilateral ABCD starts out as a trapezoid with AB||CD. Drag point D left and right keeping AB||CD.
Under the classic definition of a trapezoid (also called the exclusive definition), there is one place you could land D that would suddenly make ABCD NOT a trapezoid anymore.
Where is that location? What do you think about the fact that this shape is suddenly no longer a trapezoid?