33

33 = 3 x 11.

33 is the smallest integer that cannot be expressed as a sum of different triangular numbers (1, 3, 6, 10, 15, 21, 28, . . . ).

The smallest run of three consecutive integers that are semi-primes (the product of two primes) begins with 33: 33 = 3 x 11, 34 = 2 x 17, 35 = 5 x 7.

33 is the smallest number with nine representations as a sum of three primes: 33 = 2 + 2 + 29 = 3 + 7 + 23 = 3 + 11 + 19 = 3 + 13 + 17 = 5 + 5 + 23 = 5 + 11 + 17 = 7 + 7 + 19 = 7 + 13 + 13 = 11 + 11 + 11.

A normal human spine has 33 vertebrae.

Source: 33 Mysteries