How Many Circles Can You See In This Mind Bending Optical Illusion?

When you first look at this image, it looks like a bunch of rectangles, but it turns out there are some circles in there as well. Can you see how many?

Some people are able to spot the other shapes immediately while others have to stare for a bit, and some folks can't see them at all. Once you find them though, there is no going back. 

If you're having trouble, try looking at the vertical lines between each rectangle and it might help the circles to pop. 

It happens because of something called the Coffer Illusion, where our brain groups black, white and gray lines as either rectangles or circles. 

And there are 16 circles. Could you see them?