The great thing about geometrical-optical illusions is that you can stare and stare at them, know your brain is being fooled, and still see the illusion. So it is with the Müller-Lyer pulsating star. Believe it or not, the blue and black line segments are the same length, and they never change length. Your eye just won't believe it at first glance.
The Müller-Lyer illusion takes advantage of visual context to fool your eye and brain. A line that's bounded by closed arrow tails will look shorter than one bounded by open tails. Putting this graphic in motion makes it even clearer.
If you want a great way to kill time today, read up on the or other places of more geometric-optical illusion examples. I've always loved the , myself.