And it will be when your sons will say what is this worship of yours. And you will say this is a Passover sacrifice to G-d who skipped over the homes of the Jews in Egypt as he attacked the Egyptians thus saving our homes.

I saw a commentary of the Abarbanel on a Haggadah that explains the above passage making maximum use of the flexibility of the Hebrew words in the verse.

Unlike two other places in the Torah where the son ASKS a question, here the Torah uses the word "say" implying that this is a statement and not really a question at all. And because the word sons is plural, the Abarbanel learns that you can read the statement of this son in two different ways:

(1) What is this! This ceremony is YOURS and not a commandment from heaven!

(2) What is this! This worship is for YOU and YOUR pleasure. Since the sacrifice tastes good, you agree to do it. If it had been a less pleasant activity you would have refused!

The Torah tells the parents the proper reply to each statement:

(1) This is a Passover sacrifice to G-d - we were commanded by G-d to perform this exact sacrifice.

(2) Since we were in danger in Egypt and were saved by G-d, we owe it to Him to serve Him in whatever way He sees proper. Which means to say, had the mitzvah come in a different form, we would still perform it since we are grateful for having been saved during the Exodus.

There are two additional points to make here:

(1) Since the son said that the Passover sacrifice was not commanded from Heaven, he is not permitted to eat it since only Jews who believe in Torah Judaism can eat the Passover sacrifice. This is the idea of the Haggadah when it mentions to "inflict his teeth", in other words we are not to put a portion of the meat in his mouth.

(2) If the son does not believe in Torah Judaism he certainly won't care what the words of the Torah are saying. On the other hand, it is an historical fact that many Jews did not leave Egypt and perished there during the last plague before the Exodus. If this son had been in Egypt and did not believe in G-d, he would perished there and not exited with the Jewish people! This is the idea of the Haggadah when it mentions that had he been there he would not have been redeemed - a reply meant to "shock" him before relating to him the truth of the Torah's words.