The cup of Elijah is a problem in Talmudic law. There is debate over whether four or five cups of wine should be drunken at the Passover Seder. The verses on which the four expressions of deliverance are based apparently also contain a fifth expression of deliverance. So, we fill the fifth cup, but we don't drink from it - the perfect Jewish compromise. We call the fifth cup "the cup for Elijah." The idea is that when the messiah comes (Elijah is expected to precede the messiah), we'll find out which opinion is correct regarding drinking four or five cups at the Seder.

In some customs, each person at the Seder contributes some wine to the fifth cup, thus everyone shares in the messianic hope. In others, wine from the cup of Elijah is mixed with the fourth cup. But "Elijah" isn't explained - just used.

Others point out historically, and in my opinion correctly, that the door is opened to show everyone outside that Jews do not in any way use the blood of Christians for Passover, often called the "blood libel," but in order to reassure children and not frighten them during the Seder during the Middle Ages. To "cover" this ritual, we send children to the door to open it for Elijah the prophet. Sefardim do not open the door for Elijah in general.

  Best Wishes,

Rabbi Barry Dov Lerner