Look at the long horizontal lines. Do they look closer together at one end than at the other end or do they look parallel?
Click the button and look again.
Try moving the short lines around.
The lines look closer together on the right hand side than on the left. In fact, they are parallel — no closer on the right than on the left!
There are a number of reasons why this illusion might work.
If you want to know more about the Zollner Illusion, read the explanation.