הקהה את שנייו
Why does the answer to this son involve grinding with the teeth?
It is possible to understand the challenge of the son “what is this avodah to you”, to mean what is the purpose of the Seder? We read in a book the stories, but what do we gain from the actual experiences like eating the Matzah?
Therefore, you answer with a similarity to the question (like the saying answer a fool according to his foolishness). It is not enough to read about separation of different parts of the food, but one needs to grind with the teeth to make digestion possible. Theoretical discussions are nice, but one needs to take physical action.
Answering according to the words of the challenge is displayed by Moshe in response to Korach. Korach challenged Moshe and Aharon saying רב לכם (Bemidbar 16,3). Moshe responded (verse 7) borrowing from the same words of the challenge רב לכם.
