This magic trick demonstrates the crucial importance of measuring and comparing accurately. In science, relying solely on our eyes can be misleading. To obtain reliable results, it is essential to use rigorous measurements and appropriate tools.
Start the trick by presenting the two arches to the audience, one above the other vertically. Ask a spectator to point out which arch is smaller. After the spectator chooses, ask him to help you stretch the smaller arch until it is the size of the other one. Show the arcs vertically again, one on top of the other, but now switch the order (the one that was stretched and was initially on top is now on the bottom). Play with the audience and say, now I think we have stretched it too far.
This illusion is commonly known as the Jastrow illusion. It is an optical illusion where two identical figures are placed next to each other. Although they are both exactly the same size, one appears to be larger. When the positions of the two shapes are reversed, the impression of which is the larger is also reversed.
Scientists are not yet certain what causes one figure in the Jastrow illusion to appear larger than the other. The fact that the shorter side of one figure is next to the longer side of the other somehow tricks the brain into perceiving one shape as longer and the other as shorter, although it is unclear exactly why this is so.