A picture circulating online challenged people to determine the missing number from what looked like a simple sequence of numbers from 1 to 19. "Which number is missing?" the puzzle asks.

Number 1 is in the same location as Plutonium (Pu), Number 2 is in the same location as Sulfur (S), and 3 is where Hydrogen (H) is. So the first word of the puzzle is "PUSH." ...