The left circle has the same color pattern as the right one, but the former appears to be yellowish while the latter to be bluish. (a demo of the chromatic version of the Anderson illusion)pic.twitter.com/beLS7K61Rc
You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
Okay, I googled it. illusion by Barton L. Anderson and Jonathan Winawer
But it may be that I combined the two halves of the image myself, can't remember.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.