[Tutor] Question about order in a dictionary

[Dick Moores]

Python 2.3.4

In IDLE, if I enter
D = {"a": "first letter", "b": "second letter", "c": "third letter",
"z": "last letter"}
and then 
print D
I get 
{'a': 'first letter', 'c': 'third letter', 'b': 'second letter', 'z':
'last letter'}

Why does the order change? I'm just beginning to understand
dictionaries, and I suppose the order doesn't matter, but why would it
change?

Thanks,

Dick Moores