What number do you see?
The optical illusion works by testing an individual's contrast sensitivity, or ability to distinguish an object from its background. This is important in situatiions like driving in low light or fog, where the contrast between object and background is reduced.
In the case of the puzzle, the numbers are on one layer of the image and the swirl is on the other.
People with higher levels of contrast sensitivity are able to perceive the difference between the number and background with greater ease.
Some people will see the number 528, others 45283, and many won't see anything at all.
The correct answer, however, is 3452839 (once you know this you may be able to hunt down the numbers).
The lesson :People will often perceive the same thing *very* differently.