We all know that the Drink of Despair was one of the defenses placed for Slytherin's Locket as a Horcrux, alongside the Inferi Army and Anti-Disapparition Jinx. We also know that the Locket retrieved by Dumbledore and Harry in HBP was a fake, and that the real one was stolen long before by Regulus, who had the aid of Kreacher. 

The question is, if Regulus drank the potion and stole the real Locket and replaced it with a fake Locket, how did the Emerald Potion refill itself when Dumbledore and Harry tried to retrieve the Locket? Did Regulus know how to make the Emerald Potion or did the Potion refill itself?