As David returned from the slaughter of the Philistine, Abner took him, and brought him before Saul with the head of the Philistine in his hand.
Saul needed to know the name of David’s father to deliver the prize that he had promised to anyone who would defeat Goliath (1 Samuel 17:25). Perhaps he had never asked David about this before or had forgotten whose son he was. Another explanation of Saul’s strange ignorance is that the events of chapter 17 may have happened chronologically before those of chapter 16.