This optical illusion is a fairly simple one, but it's still a very impressive effect. A triangle and some cunningly positioned diagonal lines are all that's needed for your brain to tilt a perfect level rectangle completely off-kilter.
While this illusion doesn't feature the same level of mindbending insanity as some previous examples, it makes up for it in its sheer audacious simplicity. All it is a few shapes and some lines that disappear and reappear, and yet that's enough for the top rectangle to seem to shift from tilted to parallel with its base and then back to tilted again. The video's creator Greg Ross explains just how it works:
The sets of opposite diagonal lines in this drawing cause us to perceive the top rectangle as leaning toward the right when it really is not. Removing the lines proves that the rectangles are parallel. What also aids in the illusion is the triangle drawn on the right end of the top rectangle (we subconsciously perceive it as weighing down the right side...but only when the diagonal lines are there).
Spotted on New Scientist.