Generator oddity

[opstad at batnet.com]

I'm a little baffled by the inconsistency here. Anyone have any
explanations?

>>> def gen():
...   yield 'a'
...   yield 'b'
...   yield 'c'
...
>>> [c1 + c2 for c1 in gen() for c2 in gen()]
['aa', 'ab', 'ac', 'ba', 'bb', 'bc', 'ca', 'cb', 'cc']

>>> list(c1 + c2 for c1 in gen() for c2 in gen())
['aa', 'ab', 'ac', 'ba', 'bb', 'bc', 'ca', 'cb', 'cc']

>>> it1 = gen()
>>> it2 = gen()
>>> list(c1 + c2 for c1 in it1 for c2 in it2)
['aa', 'ab', 'ac']

Why does this last list only have three elements instead of nine?