The abnormal fear of the number 13, often known as triskaidekaphobia, is one of the most prominent myths in science and was fuelled by the failed mission of Apollo 13, which launched at 13.13 h and exploded on April 13, 1970. Some interesting coincidences, like Microsoft skipping Office version 13, are discussed. The number of observations, like 13 major joints in our body and 13 essential vitamins for our body, along with many interesting mathematical curiosities like ‘the sum of all prime numbers up to 13 is equal to the 13th prime number’ and ‘13 composite numbers between 13 and its reversal’, are discussed. In this chapter, you will see why 13 is famously unlucky, why people tend to believe in such superstition, what the effect is, and what is so interesting about the number 13.