* Starting with the number 1, you have one 1 which produces 11.
* Starting with 11, you have two 1's i.e. 21
* Starting with 21, you have one 2, then one 1 i.e. (12)(11) which becomes 1211
* Starting with 1211 you have one 1, one 2, then two 1's i.e. (11)(12)(21) which becomes 111221