void setup() {
size(320, 240);
background(153);
rectMode(CENTER);
}

int xpos = 160;

void draw(){
  
  background(204);
  
  //right
  if(mouseX > width/2){
    xpos ++;
    
    if(xpos > width){
     xpos = width; 
    }
       
  }
  
  //left
  if (mouseX < width/2){
      xpos --; 
   
       if(xpos < 0){
      xpos = 0;
    }
    
  }
 
 rect(xpos, height/2, 50, 50);
  
}