From the excellent book Quantum A Guide for the Perplexed by Jim Al-Khalili:
If you stare (below) at any one of the white circles you will notice black dots appearing and disappearing in some of the surrounding circles. But if you switch your gaze to one of these black dots it immediately turns white again and stays white. You can never catch a black dot in the act.
This simple optical illusion shows in a rough way a quantum physics idea: the act of measurement affects a quantum system.