What to see
Above you'll notice a ring of large blueish disks and a ring of small blueish disks. They slowly rotate, but that's only by the way.
In their centers are are 2 orange disks. Thry to judge whether these 2 are of equal size. They will probably not look equal – at least they don't look equal to me, the right one (the one surrounded by the small disks) looks larger to me.
What to do
Use the slider to enlarge one orange disk it relative to the other one until you're satisfied. Then, do either of 3 things:
Obviously, the context – here the surrounding smaller or larger disks – affects our size judgement.
This illusion is named after Hermann Ebbinghaus, a pioneer in memory research, who likely discovered this illusion in the 1890s, but did not publish it in any specific publication. Rather, Titchener (whithout claiming authorship) published it in a 1901 textbook; thus it is also frequently called the Titchener illusion.
The mechanisms causing this illusion are not well understood; probably it is closely related to the Delboef Illusion. It is one of the minority of illusions where a sizable age effect has been found (Doherty et al. 2010): For children under age 10 the illusion is weaker.
Doherty MJ, Campbell NM, Tsuji H, Phillips WA(2010) The Ebbinghaus illusion deceives adults but not young children. Developmental Science 13: 714–721