A straight line from corner to corner: the long sides bend, in one and out in the other.
Puzzles that fool the eye
Missing square puzzle
The same four pieces make both big triangles, but the second has a hole. Where did the extra square come from?
Neither shape is really a triangle.
Why? Neither is really a triangle. The slopes of the red and blue pieces are slightly different, so the long side bends a tiny bit, inward in one and outward in the other.
Picture: Original: Krauss Vector: cmglee, CC BY-SA 4.0 · Wikipedia(opens in a new tab)