Upright lines through the three tips: they never move, and the middle one stays halfway.
Bigger or smaller?
Brentano’s turning arrows
As the corners turn, do the two gaps between them change?
No: the tips of the corners never move, and the middle one is always exactly halfway.
Why? Franz Brentano’s version of the Müller-Lyer illusion: a gap between corners pointing in looks longer than a gap between corners pointing out. As they turn, the gaps seem to grow and shrink in turn. This is our drawing.
Picture: Drawn by Science Playground, after Franz Brentano · Wikipedia(opens in a new tab)