Optical illusions are very polarizing. Some people see it, some people don't.
The most recent one sweeping the Internet is this grid-type picture with a series of dots in it.
Your brain doesn't allow you to see all the dots at once, so it's a little tricky to spot them all. One user referred to it as "mind whack-a-mole."
How many dots do you see in this picture?
All together there are 12 dots! Most people won't be able to see them all at once, though. French scientist Jacques Nino tells us why.
"When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters which move erratically on the page. Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present. Some black sparkling can be seen at those crossings where no disk is seen. The illusion also works in reverse contrast."
So basically, it all has to do with peripheral vision. The black dot you're looking at is clear, but the ones from the corner of your eye aren't. This is because your brain has to guess what's there. Since the pattern is predominantly grey lines your brain just assumes that's the next step in the pattern, ignoring the possibility of more dots.
How many dots did you see when you first looked at the image? Be honest!