First you ask a spectator to choose a card (assuming the king of hearts) and you clearly show the bag empty that is on the table since the beginning of the trick.
You perform a magic move and introduce your hand inside the bag: you remove a card castle of 28 cm height.
You then propose to do the experiment again.
One more time you show that the bag is empty and you remove from it an identical castle.
The bag is still empty but you introduce your hand inside the bag for the last time and you remove a third card castle.
Now ask the card to the spectator and slowly return the three castles… they are only constituted by king of hearts !