sort in the list

[Shi Mu]

I use Python 2.3 to run the following code:
>>> a=[[1,2],[4,8],[0,3]]
>>> a.sort()
>>> a
[[0, 3], [1, 2], [4, 8]]
>>>
I wonder whether the sort function automatically consider the first
element in the list of list as the sorting criteria or it just happens
to be?
Thanks!