When did 'we' start Calling it World War I?

At what point during or after the First World War, did it become referred to as 'WWI'? Likewise did the second war immediately become 'WWII' or was it only called that sometime after the war ended? I realize there probably aren't too many folks on YA old enough to remember, but I'm hoping there are some Historians here who can satisfy my curiosity. Thanks!