A parallelogram looks trickier than a rectangle because it leans. But that lean is a lie — there's a clean rectangle hiding inside it, and finding it is the whole trick.
The Cut-and-slide
Take the parallelogram and slice straight down from the top corner, making a right-angle cut. That chops a triangle off one end. Now slide that triangle across to the other end, where it fits perfectly into the gap. What's left? A plain rectangle. You moved a piece around but you didn't add or remove any space, so the area is exactly the same. Press "straighten into a rectangle" in the toy and you'll watch it happen.
So the Formula Is the Rectangle's Formula
The rectangle you straightened it into is base wide and height tall, so its area is base × height. Since the parallelogram has the same area, it must be base × height too. No new rule to learn — it's the rectangle formula wearing a hat.
Why "perpendicular" Height Matters
The height has to be measured straight up from the base, at a right angle — because that's the height of the hidden rectangle. The slanted side is longer, and it's the wrong number. We'll hammer that home on rung 3, but notice it now: dragging the slant makes the side longer while the upright height never budges.