A way of saying "that's how it is" and "it is what it is." If someone says this to you, you should just accept it and move on because there is no changing the way the cookies crumble.
Person 1: I wanted pizza for supper D:
Person 2: Well thats just how the cookies crumble
Person 1: Okay, ill just accept it and move on.
"Your mom went to get the cookies" is an alternate version of "Your dad went to get the milk" or a comeback. It can also mean your mom went to get the cookies for the milk.