undefined Method

[Animatect]

Hi, I'm extending a class and then assigning an instance of this class to a variable that has the type of its superclass, when I try to access any method of this instance through the superclass typed variable, I get the Error:

1061: Call to a possibly undefined method Thirsty through a reference with static type BaseGameEntity.

please can anyone tell me what may be the problem, thanks.

I'll post a simplified version of the problem using as few classes as possible:
the error triggering code is in the Main class.

package
{
	import flash.display.Sprite;
	
	public class Main extends Sprite
	{
		private var miner:Miner;
		public function Main()
		{
		  //create a miner
		  miner = new Miner(0);

//here is an implementation that throws the error:
//----------------------------------------------

		  var a:BaseGameEntity;
		  var b:Miner = new Miner(0);

			a = b;
			
			trace (a.Thirsty());
//------------------------------------------
		
		  //simply run the miner through a few Update calls
		  for (var i:uint=0; i<20; ++i)
		  { 
			miner.Update();
		  }
		}
	}
}

package
{
	import BaseGameEntity;
	
	public class Miner extends BaseGameEntity
	{
		//the amount of gold a miner must have before he feels comfortable
		public const ThirstLevel:int = 5;
		
  		//the higher the value, the thirstier the miner
		private var m_iThirst:uint;
	
		
		public function Miner(id:uint):void
		{
			super(id);
			init();
		}
		
		private function init():void
		{	
			//BaseGameEntity(id);
			m_iThirst = 0;
		}

		public function Thirsty():Boolean
		{
			if (m_iThirst >= ThirstLevel)
			{
				return true;
			}
			
			return false;
		}

		public function Update():void
		{
			m_iThirst += 1;		  
		}
	}
}

package
{
	import flash.display.Sprite;
	
	public class  BaseGameEntity extends Sprite
	{
		//every entity must have a unique identifying number
		private var m_ID:uint;
		//this is the next valid ID. Each time a BaseGameEntity is instantiated
		//this value is updated. 
		//Esta variable es static porque queremos que el next valid ID se actualize en todas las instancias.
		private static var m_iNextValidID:uint = 0;
		
		public function  BaseGameEntity(id:uint):void
		{			
			setID(id);
		}
		
		private function setID(val:uint):void
		{

		//make sure the val is equal to or greater than the next available ID
			if (val >= m_iNextValidID)
			{
				m_ID = val;
				//esta es una static property asi que se actualiza en cada instancia.
				m_iNextValidID = m_ID + 1;
			}
			else
			{
				m_ID = m_iNextValidID;
				m_iNextValidID = m_ID + 1;
			}
		}
		
		public function IDNum():int
		{
			return m_ID;
		}  
		
		public function ID():String
		{
			if(m_ID == 0)
			{
				return EntityNames.ent_Miner_Bob;
			}
			
			else (m_ID == 1)
			{
				return EntityNames.ent_Elsa;
			}
		}  
	}
}

Thanks in advance for your help, also I cutted the code A LOT, it should be ok, but if it doesn't compile please let me know.