void setup() 
{
  size(700, 700);  
  stroke(255);     
}

float y = 300;
void draw() 
{ 
  background(0);   // Set the background to black
  if(mousePressed) {
    ellipse(mouseX, mouseY, 20, 20);
  }
  else{
    rect(0, y, width, y);  
  }
  
  y = y - 1; 
  if (y < 0) { 
    y = height; 
  } 
}