int myState = 0;

void setup(){

}
void draw(){
  if(myState==0){
    runState1(); 
  } 
  else if(myState==1){
    runState2(); 
  } 
  else {
    runDefaultState(); 
  }
}

void keyPressed(){
  if(myState<2){
    myState++;
  } 
  else {
    myState = 0; 
  }
}

void runState1(){
  background(255,0,0);
  rect(random(100),random(100),10,10); 
}
void runState2(){
  if(mousePressed){
    rect(random(100),random(100),10,10);
  }
}
void runDefaultState(){
  background(0,255,0);
  rect(random(100),random(100),10,10);
}