Make it with bottom, second, and third decks. (Genesis 6:16)
The Torah tells us that Noah was instructed to construct the Ark with three levels.
Since we know that it took Noah 120 years to build the Ark (see Rashi to 6:14), we can assume it took him approximately 40 years of effort per level. Now, what was the purpose of each floor of the Ark?
The Talmud (Sanhedrin 108b) relates that the bottom level was for refuse and waste products; the middle level was for animals; the top level was for Noah and his family.
We can easily justify why he needed to spend 40 years of work to construct a dwelling place for the animals and 40 years for his family, but why did Noah have to spend 40 years of his life building a level of the Ark to keep the animal waste? Why couldn’t he simply have thrown it overboard?
I heard a great and original answer to this question from Rav Ilan Feldman, shlita, of Atlanta, Georgia.
Whenever a person undertakes a worthwhile cause or project, there are always desirable parts of the task, and also what is commonly referred to as “the dirty part of the job.”
For example, a person who undertakes to help a Yeshiva organize a gala fundraising dinner will find that welcoming the guests on the evening of the event and addressing the attendees are some of the more enjoyable and honorable parts of the job.
On the contrary, calling hundreds of people in the weeks beforehand is a difficult and somewhat thankless task.
The question is: what should be a Jew’s attitude towards the latter?
The answer is that we learn from Noah how we should respond. Instead of rejecting this type of “dirty work,” we have to embrace it and understand that it too is part and parcel of Avodas Hashem (serving G-d).
Noah was faced with precisely this type of dilemma. Undoubtedly, saving the world and humanity were exciting propositions, but tending to the physical needs of elephants and gorillas was far from glorious.
Noah wasn’t permitted to throw the waste overboard. Instead, he had to spend 40 years building a level of the Ark to retain it – to teach us that there is no such thing as dirty work in serving our Creator and His creations.