OK, this is very simple:
1. Stare at the red dot on the girl's nose for 30 seconds [if you left-click, you can enlarge the image].
2. Turn your gaze to a plain surface such as a wall or the ceiling.
3. Blink repeatedly and quickly.
I like this not only for the simple thrill of having the external world filtered in an unusual way, but more profoundly because it is yet another reminder that our sensory organs are finely pitched to a specific perceptual band and that they work in a highly-evolved though far from 'objectivity'-inducing manner. Just as dogs hear within their own amplified frequency range and pick up acutely on scents that 'us humans' do not notice, so they fail to see the range of colours that 'we' 'see'... No wonder I used to spin myself round and round as a child until I got dizzy. Slippery slope.