unity_dropping_tile.cs

// Create a floor tile, and put this script on it.
// Make sure that floor tile has a collider and a rigidbody.
// Make sure that rigidbody has `Use Gravity` but not `Is Kinematic` checked
// Also make sure it has all of the constraints checked
 

using UnityEngine;
using System.Collections;

public class dropTile : MonoBehaviour {
	public bool shouldDrop;
	
	void OnCollisionExit (Collision c) {
		Debug.Log("EXIT COLLISION", gameObject);
		if (shouldDrop == true) {
			gameObject.GetComponent<Rigidbody> ().constraints = RigidbodyConstraints.None;
		}
	}
}