float x, y;     //screen position of the particle
float vx, vy;   //velocity of the particles

void setup() {
 size(400, 400); 
 stroke(0);
 strokeWeight(3);
 fill(150);
 smooth();
 //set position and velocity
 resetParticle(); 
}

void draw() {
  background(255);
  
  //draw the particle with diameter of 10
  ellipse(x, y, 10, 10); 
 
  /*
   * update the position by adding on the velocity
   * remember that velocity means incremental change
   * in position.
   */
  x += vx;               
  y += vy;
  
}

// on a mouse click
void mousePressed() {
  resetParticle();
}

void resetParticle() {
  //reset the particle to the middle of the screen
  x = y = width/2f;    
  
  //set new random velocity
  vx = random(-0.39, 0.1);
  vy = random(-0.1, 0.21);
}