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!