Here's my Pesach D'var Torah as told to me by my wife. Feel free to use it at your own Seder.

As you all know we drink 4 cups of wine at the Seder. And we also place a fifth cup on the table, which we call the Cup of Eliyahu (Elijah).

(The fifth cup is the one that someone at the table always shakes when we open the door so that the little kids at the table think that Eliyahu is drinking it. You know what I'm talking about.)

The reason we drink 4 cups is for the phrases of redemption listed in the Torah: (1) I released you, (2) I saved you, (3) I redeemed, and (4) I took you.

But there is actually a fifth phrase that is also connected to the story of Pesach: "And I brought you in...".

There is a disagreement whether the 5th phrase is part of the 4 or not.

It it is, then we should drink 5 cups, and not 4 (could you even handle 5?).

Since the Rabbis didn't have an answer, they decided to leave the question unanswered... until Eliyahu Hanavi returns to answer the question about the fifth cup.

Hence the fifth cup is called the Cup of Eliyahu.

Pretty Cool.