Why does use of nonalphanumeric characters make a password more difficult to crack?

What will be an ideal response?

Using nonalphanumeric characters makes a password more difficult to crack because it adds to the number of possible unique combinations over and above using only alphanumeric characters. This is also true because most password crackers use combinations of common words and numbers to break a password, and adding nonalphanumeric characters into the mix introduces less common combinations, and it is more difficult to crack.