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זכר למקדש כהלל: כן עשה הלל בזמן שבית המקדש היה קים. היה כורך פסח מצה ומרור ואוכל ביחד. לקים מה שנאמר: על מצות ומרורים יאכלהו.
In memory of Hillel: This is how Hillel stood in the times of the Temple. There was a sacrificial lamb, which we substitute with Matzah, Maror and Food [Charoset] together. Keeping what was written: "On the Matzot and Marors He will eat them."
During this part of the Seder, we create a sandwich made of Matzah, Maror and Charoset.
This tradition comes from the description in Shemot that tells us to eat the Pascal Lamb Offering. Today as we do not sacrifice animals, we create this sandwich instead. We are supposed to eat starting from the bitter side and ending to the sweet, to remind us that slavery was bitter yet the outcome of freedom is sweet.
Hillel believed that the Matzah and Maror should be eaten together, while other sages believed that they should be eaten separately. This part of the Seder fixes that disagreement: they are eaten separately beforehand and then are together during Korech.
In the times of the Temple, there were two opinions on how to eat the substitution for the sacrificial lamb. The majority ruling was to eat the Matzah, Maror and Charoset separately, yet Hillel believed that they should be eaten all together. In order to compromise with each other, they created a section within the Seder for the Matzah and Maror to be eaten separately, and a section in the Seder for them to be eating together as a whole.
Why do some sages believe that the matzah and maror should be eaten separately?
In Berachot 49a in the Talmud, it states that we shall not perform mitzvot "bundled together" ("chavilot chavilot"). This is because we do not want to give the impression that mitzvot are an unwanted burden. This is an obligation that we wish to always complete. For this reason, the majority opinion is that the two mitzvot of eating matzah and maror should be performed separately.
Instead of a sandwich made out of Matzah, Maror and Charoset, the new sandwich will be a little more modern and applicable for today. The new sandwich will be made out of Reese's, Oreos, and raspberry chocolate. The bottom will have matzah with the frosting from one Oreo. Then one Reese's cup will be put on the matzah and frosting. The top of the sandwich will have matzah with the frosting from another Oreo. Finally, it will be covered with melted raspberry chocolate. This new sandwich will be more modern and even more tasty. And it will even be kosher for Pesach!