Its first digit is its second digit squared.
It's divisible by 2.
It's divisible by 3.
It's divisible by 6.
It's divisible by 7.
It's divisible by 14.
It's divisible by 21.
It's divisible by 1 and by 42.
It's the product of 6 and 9 (in base 13).
It's a composite natural number, bracketed by a twin prime, whose factorization makes it a sphenic number.
It's also a pentadecagonal number, a Catalan number, a meandric number, an open meandric number and a Harshad number.
According to Douglas Adams, it's the answer to everything.
It's the sum of the numbers/spots/pips on a pair of conventional (six-sided) dice.
It's the number of teeth a dog has.
On the telephone keypad it spells "ha."
It's been my favorite number, for reasons I have never known, since I was a child.
It's the atomic number of molybdenum.