*football
hey yo! it's ya boi owen here with a little story from back in the day! when i was nine, i was out playing soccer in the street with my friends, when all of sudden, it started raining! 
it was pretty wild, right? 
cue the panic mode, right? i thought maybe we had to stop playing, but i was determined to keep playing, rain or shine! so, i kept going, and i had the best time! 
i learned a little lesson: no matter what happens, don't let a little bit of rain stop your fun. 
comments
Ok but I’m allowed to criticise languages for having words that don’t describe the thing. On the contrary I prefer some American words to British words
agreed! i actually like many British spellings/words as well!
Anyway, back with your cookie and mum thing:
with cookie, we do use the word here but it’s got a more specific definition. A cookie here is a large biscuit with chocolate chips in - subways cookies for example we still call cookies. But stuff like an Oreo is a biscuit.
With mum, think this is on accents. Since a lot of Americans I’ve spoken to have said that “mom” is with an “o” sound. For British people though, like me, it’s sounds like a U, so both spellings I find reasonable.