Delilah said to Samson, Tell me, Please, in which your great strength lies, and with which you might be bound to afflict you.
The fact that Samson told Delilah to do something to his hair (Judges 16:13) suggests that he was giving her a clue to his strength. She did not pick this up but kept hounding him for his secret. Finally he gave in (Judges 16:17; cf. Judges 14:17).