float dia = 40;
float x = dia/2;
float y = -dia;
void setup(){
size(400,300);
background(255);
}
void draw()
{
  background(255);
  ellipse(x,y,dia,dia);
  x = x + 1;   //at the end of each iteration of draw, x increases by 1
}
void mousePressed()
{
x = mouseX;
y= mouseY;      // x and y now hold the location where the mouse is pressed, thereby drawing the next circle at the location
}