On The Big Bang Theory, Sheldon Cooper called 73 the best number. It sounds like a joke — but the math behind it is surprisingly beautiful. First, 73 is a prime number. But it also holds a special spot in the list of primes. If you count them one by one, 73 turns out to be the 21st prime number. Now flip the digits of 73 and you get 37 which is the 12th prime number (21 reversed becomes 12). Second, take the number 21 and factor it: 21 = 7 × 3. Those are the exact digits that make 73. And finally, write 73 in binary (the language computers use): 1001001. Read it left to right or right to left — it’s the same. That makes it a binary palindrome.

'The Big Bang Theory' enters the multiverse with 'Stuart Fails to Save the Universe,' which finds Comic Center of Pasadena owner (Kevin Sussman) on a journey through space and time after a device built by Leonard Hofstadter and Sheldon Cooper goes haywire!