Mensa 'Squares' Algebra Problem

 The number 24 can be written as a difference of two squares in two different ways, i.e.

7 ²   -   5 ²

And:

 5 ²  -    1 ²

 The number 21 can also be written as a difference of two squares in two different ways, i.e.

 11 ²  -    10 ²

And:

 5 ²  -    2 ²

Can all positive integers be written as the difference of two squares in at least one way?

If not, which ones can?