public void DamageTest()
    {
        float       damage      = 10;
        EarthHealth earthHealth = new EarthHealth();

        float startHealth = earthHealth.GetHealth();

        earthHealth.TakeDamage(damage);
        float newHealth = earthHealth.GetHealth();

        Assert.AreEqual(startHealth, newHealth + damage);
    }
    void Start()
    {
        GameObject earth  = GameObject.FindGameObjectWithTag("Earth");
        GameObject player = GameObject.FindGameObjectWithTag("Player");

        playerHealth = player.GetComponent <PlayerHealth>();
        playerWeapon = player.GetComponent <PlayerWeapon>();
        scoreKeeping = player.GetComponent <ScoreKeeping>();
        earthHealth  = earth.GetComponent <EarthHealth>();
        upgradeMenu  = gameObject.GetComponent <UpgradeMenu>();

        if (upgradeMenu != null)
        {
            RefreshTexts();
            ToggleUpgradeButtons();
        }
    }