Print is distressed for a vintage appeal Officially Licensed "The best number is 73. Why? 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, 1001001 which backwards is 1001001." This episode is the 73rd episode in the entire Big Bang Theory series.