I am creating a small piece of code where i will be able to make raindrops fall on a building and when a raindrop hits the building i want to make a holes in the building. so i want it to erode. If you can see if i have started on the first building but i am stuck as i cant go past the first line.

test.pde

Drop[] drops = new Drop[1000];  // An array of drops
int totalDrops = 0;            // New variable to keep track of total number of drops we want to use!

int x = 600;
int y = 75;
int w = 100;
int h = 100;
boolean button = false;


void setup() {                       // this is called once at the start of the program
 size(700,330);                     // make a window 400,400
 smooth();                          // make the drawing smooth (anti-aliased)
}

void draw() {                        // this is called repeatedly as the program runs (e.g. 25 times per second)

frameRate(100);
if (button) {
  background(0);
  building();
  windows();
  rain();
  stroke(255);
} else {
  background(255);
  building();
  windows();  
  stroke(0);
}
  fill(175);
  ellipse(x,y,w,h);
  //ellipse(340,5,10,10);
}

void rain() {
 drops[totalDrops] = new Drop(); // Initialize one drop
 totalDrops++ ;                  // Increment totalDrops

// If we hit the end of the array
 if (totalDrops >= drops.length) {
   totalDrops = 0; //Start over
 }
 // Move and display drops
 for (int i = 0; i < totalDrops; i++ ) { // New! We no longer move and display all drops, but rather only the “totalDropsâ€&#65533; that are currently present in the game.
   drops[i].move();
   drops[i].display();
 }
}

void building() {

 fill(150);
 rect(38,166,154,168); //1st
 rect(248,150,20,180); //2nd left
 rect(408,150,20,180); //2nd right
 rect(256,70,162,80); //2nd mid-top
 rect(266,132,142,198); //2nd middle
 rect(266,58,142,12);  //2nd top thin
 triangle(408, 58, 340, 5, 268, 58); //2nd top
 rect(476,230,185,102); //3rd top
 rect(488,168,159,62); //3rd bottom
 //ellipse(340,5,10,10);
}

// When the mouse is pressed, the state of the button is toggled.  
// Try moving this code to draw() like in the rollover example.  What goes wrong?
void mousePressed() {
 if (mouseX > x-50 && mouseX < x+w-50 && mouseY > y-50 && mouseY < y+h-50) {
   button = !button;
}  
}
void windows() {

 for (int i=42; i < 178; i +=15){
   for (int j=315; j > 155; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
 for (int i=272; i < 400; i +=15){
   for (int j=315; j > 115; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
 
 for (int i=260; i < 402; i +=14){
   for (int j=113; j > 60; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
 
 for (int i=252; i < 415; i +=161){
   for (int j=315; j > 135; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
 
 for (int i=495; i < 645; i +=15){
   for (int j=210; j > 150; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
 for (int i=480; i < 650; i +=15){
   for (int j=315; j > 215; j -=20){
     fill(random(0),random(255),random(255));
     rect(i,j,10,15);
   }
 }
}

Drop.pde

class Drop {

 float x,y;   // Variables for location of raindrop
 float speed; // Speed of raindrop
 float r;     // Radius of raindrop

 Drop() {
   r = 4;                 // All raindrops are the same size
   x = random(width);     // Start with a random x location
   y = -r*2;              // Start a little above the window
   speed = random(1,5);   // Pick a random speed
 }

 // Move the raindrop down
 void move() {
   // Increment by speed
   y += speed;
 }

 // Check if it hits the bottom
 boolean reachedBottom() {
   // If we go a little beyond the bottom
   if (y > height + r*4) {
     return true;
   } else {
     return false;
   }
 }

 // Display the raindrop
 void display() {
   fill(50,100,150);
   noStroke();
   for (int i = 1; i < r; i++ ) {
     ellipse(x,y + i*4,i*2,i*2);
     // rect(38,166,154,168); //1st
   }
   if ((x > 38) && (x < 190) && (y > 166)) {
     y=166;
     fill(0);
     rect(x,y,7,10);
   }
 }
}

Thanks for any help in advance!