It’s like algebra for the eyes. Viral brain teaser enthusiasts are straining their brains trying to decipher this hypnotic optical illusion, which conceals a mystery number. “We are celebrating a big birthday and it’s the number hidden in these images,” said Alex Fagelson of Jackpotjoy, which created the stigmatism-inducing image in honor of the online casino’s birthday. According to Jackpotjoy,...