This new number puzzle is going viral on Facebook and it is tricking thousands of people. The challenge looks very simple at first glance. You are given 20 numbers in random order and you have to find which number from 1 to 21 is missing. The grid shows 19, 2, 12, 4, 20, 15, 7, 8, 14, 10, 17, 16, 9, 6, 21, 3, 11, 18, 1, and 5. Most people scan quickly and think all numbers are there.

The trick is that your brain tries to read the numbers instead of counting them in order. When numbers are scrambled like this, it is very easy to miss one in the middle. The best way to solve it is to mentally count from 1 to 21 and check each one off. Start with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on. If you do it slowly, you will see the gap.

The missing number is 13. Yes, 13 is the only number from 1 to 21 that does not appear in the image. Many people guess 0 or 22 because they think outside the box, but the answer is right in the middle. The number 13 is missing between 12 and 14, and most eyes skip right over it because the grid is designed to confuse you.

Did you find 13 on your first try? If you did, you have a very sharp eye and a high IQ. This puzzle proves how our brain can overlook the simplest things when they are hidden in disorder. Share this with your friends and family and see how fast they can spot the missing number.