The little old man saw the sons' sad frowns.  He asked kindly, "What's the matter?"

They replied, "Our father is the king.  He is seriously ill.  No one can find a cure at all."

The little old man replied, "The Water of Life would cure him.  This drink cures all illnesses.

However, it's very hard to find.  I am sorry I don't know where to find it myself."