וְהָיָ֕ה כִּֽי־יֹאמְר֥וּ אֲלֵיכֶ֖ם בְּנֵיכֶ֑ם מָ֛ה הָעֲבֹדָ֥ה הַזֹּ֖את לָכֶֽם׃
And it shall come to pass, when your children shall say unto you: What mean ye by this service?
Read kabbalistic (mystical) interpretation of Exodus 12:26: and classical mystical sources.