"73 is the 21st prime number. Its mirror 37 is the 12th prime number and its mirror 21 is the product of multiplying (hang on to your hats) 7 and 3"
"In binary 73 is a palindrome 1001001 which backwards is 1001001 exactly the same."
Sheldon doesn't mention that the 73th day of a non-leap year is March 14, and this is Pi Day and the birthday of Albert Einstein.
Or that 73 is the alphanumeric value of the word NUMBER: 14 + 21 + 13 + 2 + 5 + 18 = 73.
Or that 73 is the smallest multi-digit number which is one less than twice its reverse