Optional parameters in functions and their mutable default values

Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument

I'm kind of confused about how optional parameters work in Python functions/methods.

I have the following code block:

>>> def F(a, b=[]):
...     b.append(a)
...     return b
...
>>> F(0)
[0]
>>> F(1)
[0, 1]
>>>

Why F(1) returns [0, 1] and not [1]?

I mean, what is happening inside?