float xd = 2;
float yd = -1;
float zd = 1;
float leftwall = 300;
float topwall = 250;
float backwall = 0;
float xwidth = 410;
float ywidth = 310;
float zwidth = 210;
float xpos = 0;
float ypos = 0;
float zpos = 0;

void setup() {
  size(800,600,P3D);
  frameRate(15);
  
}
void draw() {
  lights();
  background(250);
  stroke(0,0,255);
  
  translate(leftwall,topwall,backwall);
  
  noFill();
  box(xwidth, ywidth,zwidth);
  balloon();
  }
void balloon() {
 moveballoon();
 translate(xpos, ypos, zpos);
 fill(0);
 stroke(255,0,0);
 sphere(10);
}
void moveballoon() {
  xpos +=xd;
  ypos +=yd;
  zpos +=zd;
  if (xpos<-.5*xwidth) xd=-xd;
  if (xpos>.5*xwidth) xd = -xd;
  if (ypos<-.5*ywidth) yd=-yd;
  if (ypos>.5*ywidth) yd=-yd;
  if (zpos<-.5*zwidth) zd=-zd;
  if (zpos>.5*zwidth) zd=-zd;
}