What to see
After the initial animation you see two towers on the right. They do not seem to be parallel, the right one leans to the right.
What to do
If you press the button ‘move’ then the two towers will slide on top or each other. Obviously, the two towers are identical! So how can they lean?
Press ‘move’ again to disengage.
You might want to try to adjust the angle of the left button with the circular slider, but when they seem roughly parallel the aren't when moved on top of each other.
This beautifully simple effect was reported by Kingdom et al. in 2007. As they write in their paper “The explanation for this illusion would seem to be straightforward”: If in reality we see two towers next to each other (see the lower figure for the full picture), all elements of the figure must have the same perspective vanishing point. When we simply duplicate one tower and translate it, this also translates the vanishing point for the duplicated tower. So, if in reality we would see two adjacent towers with different vanishing points, then they would acutally be leaning. So one could say this is no illusion after all!
The original version, using the leaning tower of Pisa, won first prize in the Illusion-of-the-year contest 2007.
My tower example happens to be the Sheraton in Chongqing. While it looks leaning, it isn’t, that’s just a consequence of camera position relative to the center combined with the laws of perspective.