New Delhi, Aug. 4 -- A new optical illusion is going viral online, forcing people to put on their thinking caps to solve the puzzle. It shows three straight horizontal lines stacked vertically and asks a basic question: which one is the longest? The challenge is to give the correct answer in 15 seconds.

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This type of riddle is not new, but the simplicity makes it effective. The spacing between the lines and their position on the screen create a false sense of difference. Without a ruler, it is easy to assume one line stretches further than the others.

That instinct is what optical illusions like this aim to test. They are n...