Then David ran, and stood over the Philistine, and took his sword, and drew it out of the sheath of it, and killed him, and cut off his head therewith. When the Philistines saw that their champion was dead, they fled.
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.