Before you move on too fast thinking we are going round in circles. Use your mouse and click on one of the corners of the square, then drag. You can move it about because it is not a static image but interactive geometry.

So, is this a square? The issue to be resolved is not one of measurement any more but of observation. Do all four sides remain equal to each other. Do all four angles remain right angles?

Once again, they look like they might be. But to be sure, we have to interact with real live versions of these quadrilaterals.