Learning to declare functions

How come this could does not operate correctly? Im practicing writing functions, yet the serial monitor continously returns zero. What have I done incorrectly? Your help is very much appreciated. Cheers!

void setup(){
  Serial.begin(9600);
}

int in = 0;

void loop(){
  hippocampus(in);
  Serial.println(in);
  amygdala(in);
  Serial.println(in);
}

int hippocampus (int x){
  x = 1000;
  return x;
}

int amygdala (int y){
  y = 2000;
  return y;
}

Serial.println(hippocampus(in));

Serial.println(amygdala(in));

LOLLLLLLLLLL

Thank you

  hippocampus(in);
  Serial.println(in);

That didn't change "in". An alternative would have been:

void hippocampus (int & x){
  x = 1000;
}

Now that actually changes whatever-it-is you pass as an argument, which you can the print as a separate operation.