Mah Nishtanah, meaning what is made different, is a phrase known to anyone whos been to the first 10 minutes of a Seder night.
It has even entered common usage as an ironic way of questioning unusual behaviour; to someone whos habitually late but for once turns up on time, you might ask Mah Nishtanah?
Mah Nishtanah is the opening of the Four Questions (five if you count Mah Nishtanah itself) traditionally asked by the youngest child present at the Seder. The questions point out four obvious ways in which this night is different from all other nights. If there are no children, the grown-ups ask one another. If you are alone, you should ask yourself (Talmud Pesachim 116a).
Provoking questions is essential to the Seder. In fact, the text of Mah Nishtanah is a sort of fallback plan thats necessary only if no one asks.
We are supposed to do something different, or surprising, on Seder night to get the children to ask in spontaneous amazement and without the need for a script. Some rabbis removed the table before dinner or gave special treats to the kids to stimulate wonder and engender new questions (Pesachim 115b). A good question shakes us out of fixed routines of thought and opens us up to genuine learning.
When we really feel how different this night is, we are ready to hear the Hagadahs answer to why it is that God intervened to save our ancestors from the crushing routine of slavery, and introduced different principles of freedom and human dignity to be motive forces of history.