Sheldon really loves the number 73. While he didn’t explain his reasoning until the show’s seventy-third episode – ‘The Alien Parasite Hypothesis’ – the clues had been present for quite a long time. This was particularly clear in just how many t-shirts Sheldon owns with the number on it.

So, what’s so special about 73? In Sheldon’s own words: "73 is the 21st prime number. Its mirror, 37, is the 12th and its mirror, 21, is the product of multiplying 7 and 3”. In binary, 73 is a palindrome too and it’s also the year in which Jim Parsons was born! Neat, right?