Number Riddle

I've always been a fan of this riddle. Although not truly math related, the students will assume it is because of the numbers.

Examine carefully the following sequences of numbers:

1

11

21

1211

111221

312211

13112221

1113213211

31131211131221

Although the sequences appear to behave totally at random, in fact, after the first sequence, each sequence is constructed in a precise and methodical way based on the previous one. What is the next sequence?

To solve, simply say the quantity of each number. Ex. in the first line, there is one 1, hence why the second line is 11. Now, the second line has two 1's, hence what the third line is 21.

Get it?