One trick someone suggested is:
Hold the key in a flame till it is quite hot, then stick it into the
frozen lock.

I used to own a car in a place where it got cold enough to try that.
I remember that it worked, but I can't remember whether it _always_ worked.

--
Wes Groleau

"What progress we are making! In the Middle Ages, they would have
burnt me; nowadays they are content with burning my books.€�
€” Sigmund Freud, 1933
"He was never to know that even that was only an illusory progress,
that ten years later they would have burned his body as well.€�
€” Ernest Jones, 1953