var ballnum = 0;

function ChangeBall() {
  // Increment the ball number.
  ballnum = ballnum + 1;
  // Only five different numbers allowed.
  if (ballnum > 5) {
    ballnum = 1;
  }
  // Choose an appropriate color
  if (ballnum == 1) {
    document.ball.src = "red-ball.gif";
  }
  else if (ballnum == 1) {
    document.ball.src = "green-ball.gif";
  }
  else if (ballnum == 3) {
    document.ball.src = "blue-ball.gif";
  }
  else if (ballnum == 4) {
    document.ball.src = "cyan-ball.gif";
  }
  else if (ballnum == 5) {
    document.ball.src = "yellow-ball.gif";
  }
  // Prepare to do it all over again
  setTimeout("ChangeBall()", 300);
} // ChangeBall()