and when the side lengths are just right, we have a square - wow- that's a parallelogram too! Same formula.
Keep the focus on the height - this will pay off soon. We can show this with guide lines, and through demonstration or student exploration with a dynamic geometry tool such as GeoGebra. We draw a bounding rectangle and show that so long as the height and base stay unchanged, the area is also unchanged.Now we focus on what happens to the side lengths as we change the angles while the height remains constant. What happens to the side length? How does it compare to the height?.
My students liked this version: "All crows are birds, but not all birds are crows". This is time for a discussion of 'all squares are rectangles, but not all rectangles are squares". We look at squares as an aside - so it's clear the square is 'just' a special rectangle.Understanding what area is, and how it is different from length and volume is non-trivial - it's quite a deep teaching - and well worth quality time. Rectangles make sense to most students, the math is very simple - so we can focus on the real meaning of area as a measure of 2 dimensional space.
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