Level lines through the joints: they never move.
Bigger or smaller?
Müller-Lyer in motion
Watch the red and blue segments. Do they stretch and shrink?
No: every joint stays exactly where it is. Only the arrowheads move.
Why? The Müller-Lyer illusion, set in motion: a segment between arrowheads pointing out looks shorter, and between arrowheads pointing in, longer. As the arrowheads flip, the segments seem to stretch and shrink and the lines to ripple. Gianni A. Sarcone made the first; this is our drawing.
Picture: Drawn by Science Playground, after Gianni A. Sarcone · Wikipedia(opens in a new tab)