This video published on YouTube by user BigKnows will really test your vision.
It may not be scientific, but it has certainly proved very difficult and frustrated many people online who just couldn't figure it out.
The video consists of letters or numbers, with one or two supposed to be different. It sounds simple enough, but it's actually not easy, so don't be fooled - as I was.
It may also be interesting if you wear glasses to try it with them on, then without and then with them again.
Don't forget to share this test and challenge your friends!