private void OnTriggerEnter2D(Collider2D other)
    {
        // respawn
        if (other.gameObject.CompareTag("DeathPlane"))
        {
            LoseLife();
        }

        if (other.gameObject.CompareTag("Bullet"))
        {
            TakeDamage(1);
        }

        if (other.gameObject.CompareTag("Gem"))
        {
            manager.addScore(10);
            manager.CollectedHeart();
            other.gameObject.SetActive(false);
            HealDamage(10);
        }
    }