Python list manipulation differs based on initialization

I want to have square matrix of integers with dimensions 3×3. I don’t want to use numpy. There are several ways how to initialize such structure. For example:

Direct definition (explicitly written out the whole structure):

a = [ [0,0,0], [0,0,0], [0,0,0] ]

and

b = [[0]*3]*3

When I type ‘a’ or ‘b’ in python3 interpreter I got same results (str function for both objects prints out the same):

>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> b
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]

however, when I try to set for example number on position row, column = 0, 1 I get different results:

>>> a[0][1] = 1
>>> b[0][1] = 1
>>> a
[[0, 1, 0], [0, 0, 0], [0, 0, 0]]
>>> b
[[0, 1, 0], [0, 1, 0], [0, 1, 0]]

How so? What’s special about [[0]*3]*3?