A good question that deserves a good answer!
The best answer I have heard is the following. The “Chida” (Rabbi Chaim Yosef David Azulai) explains that there are three ways one can fulfill the mitzvah of the telling of the story of the Jewish People’s exodus from slavery in Egypt. The first way, which is the ideal, is to tell the story in the form of questions and answers. Our Talmudic Sages derive this from the Torah's description of matzah as "lechem oni", which translates as “the ‘bread’ over which a person answers (questions about what’s going on here on this special night).”
The second level is to tell the story even if nobody asks. This is derived from the verse "and you shall tell your son on that day..." This means that you should tell him even if he doesn't ask. Thus, the procedure of question and answer is preferable, but not absolutely necessary. (By the way, this is a rare example of l'chatchila (preferred) and b'dieved (acceptable, even though not preferred) in a Torah mitzvah.)
The third level is this: Even if a person is alone he must speak about the going out of Egypt. We say in the Passover Haggadah that Rabban Gamliel teaches that one must "say" three specific statements as the bare minimum to fulfill the mitzvah. These three are “Pesach, Matzah, Marror” — the Passover offering, eating unleavened bread and eating bitter herbs.
So we see that there are three possible levels on which to perform the mitzvah of "sippur yetziat Mitzrayim — i.e., telling of the story of the Jewish People’s exodus from slavery in Egypt.
The Chida adds that when introducing the fourth son — the one who doesn’t know how to ask any questions about Passover — the Haggadah uses the word "and." This teaches us that even if someone has other children who fit into the first three categories, he must also pay attention to the one who does not know how to ask. This is an important lesson for those who are tempted to make the Passover Seder an intellectual display that goes over the head of the youngest or least knowledgeable.
May you have lots of good questions and answers on Passover!
