Circles through the joints: they never move, so every piece keeps its length.
Bigger or smaller?
Sarcone’s pulsating star
Do the blue and black parts of the star stretch and shrink?
No: the blue and black parts are always the same length. Only the arrowheads change.
Why? Another Müller-Lyer illusion in motion: as the arrowheads flip, each part seems to grow and shrink, so the whole star seems to pulse. 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)