Instead of answering Samson’s riddle outright, they give him a riddle of their own. They are imitating his riddle, and thereby mocking it and rubbing it in his face that the got the solution to his riddle. By stating their answer in interrogative form, the Philistines imply that any fool would have been able to solve Samson’s riddle.
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18 And the men of the city said to him on the seventh day before the sun went down, “What is sweeter than honey? What is stronger than a lion?”And he said to them, “If you had not plowed with my heifer, you would not have found out my riddle.”