Example of global state script in unity. Doesn't need to be attached to any object, just call its methods and read its properties.

GlobalState.cs

using UnityEngine;

public class GlobalState {
	// bool paused = false;
	public static int goop = 80;
	public static int waveNumber = 1;
	public static bool paused = false;

	public static void AddGoop (int amount) {
		goop += amount;
	}

	public static bool ConsumeGoop (int amount) {
		if (amount <= goop) {
			goop -= amount;
			return true;
		}
		else {
			Debug.Log("Cant afford " + amount + " for that");
			return false;
		}
	}

	public static bool SetPaused (bool pause) {
		paused = pause;
		return paused;
	}

	public static void WavePlusPlus() {
		waveNumber++;
	}

}