How many right angles appear in the following image?

Which arc comes from the largest circle?
If you cover the left and right edges you will see that they are all the same size. This is because the size of objects is sometimes distorted based on the context in which they are seen.

This figure has conflicting depth cues, so it becomes unstable as you look at it. Depending on which part looks closer at the moment, you should be able to see a chair-like figure, or a floating block.

Are the points marked with A and B in front, or in back?

There are no depth cues, so its impossible to tell. As a result, the image becomes unstable and may appear to flip from front to back as you continue to look at it.