Pesachim 39 - Bitter herbs
Together with the Passover lamb at the Seder one must eat bitter herbs . Now that there is no Temple and no Passover offering, strictly speaking bitter herbs are not required, but the Sages instituted that they should be eaten nevertheless, so that this mitzvah will not be forgotten.
What kind of bitter herbs should one eat? Any lettuce will do, but people usually use romaine lettuce. Although it is not really bitter, it would become so if let to grow and become old, and that is enough. Actually, any bitter herb will do, but the Talmud enumerates a few specific ones, because their names contain hints. For example, "chazeret," or lettuce is "chasa" in Aramaic, because God "chas" - pitied us. Horseradish can also be used, but extreme bitterness is not required, as shown by lettuce. In fact, the quality of lettuce being sweet at first and bitter later reminds us how Egyptians treated the Jews: with soft talk first, but harsh actions later.
How do we know that more than one type of herb is valid? - Because it is similar to matzah, which also can be made of different types of grain. The bitter vegetable must not be boiled or pickled, because this takes away the bitterness.
Art: The Bitter Draught by Adriaen Brouwer