New Delhi, Aug. 30 -- An optical illusion shared on Reddit is making people stop, stare, and even argue. The viral post, uploaded by user rastroboy on the subreddit r/opticalillusions, has already attracted more than 2,000 comments as people try to answer one simple question: "What number do you see?"

At first glance, the image looks like a dizzying set of swirling black and white circular lines. But hidden within those curved lines is a sequence of numbers. The catch? The numbers don't reveal themselves immediately. You have to focus carefully to make them appear one by one.

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Viewers see different numbers depending on how long they look and...