function updateScore(){
      currentScore += 5;
      score.innerHTML = 'Score: ' + currentScore;
    }
    
    function difficulty(){
      //as the game progresses, increase magnitude of the vertical speed
      if(currentScore % 1000 == 0){
        if(dy > 0)
          dy += 1;
        else
          dy -= 1;
      }
    }