From Rectangle to Square

#####Why is a Square a Rectangle?

Why is a square a rectangle? Formally, a square has all the properties of a rectangle; but how could you explain it to a child in a convincing way?

Here is a design inspired by classroom teachers: Starting from a rectangle, we could create infinitely many rectangles, ..., arriving, at some point, at a square. Therefore, a square is a type of rectangle. So, in a real sense, a square is a stop in a dynamic progression of rectangles.

Two versions are included. The inner square is both 50mm by 50mm. Version A can be smoothly separated; version B has a built-in groove and provides some resistance but can also be taken apart and reassembled. Have fun playing with math!