void CreateObstacle(){
        // generating random upper pipe position
        float randomPos = 4f-(4f-0.8f-pipeHole)*Random.value;
        // adding upper pipe to stage
        GameObject upperPipe = (GameObject)Instantiate(pipeObject);
        // setting upper pipe position
        upperPipe.transform.position = new Vector2(4f,randomPos);
        // adding lower pipe to stage
        GameObject lowerPipe = (GameObject)Instantiate(pipeObject);
        // setting lower pipe position
        lowerPipe.transform.position = new Vector2(4f,randomPos-pipeHole-4.8f);
    }