(10) When you have eaten your fill, give thanks to your God ה' for the good land given to you.
From Wikipedia:
Birkat Hamazon is made up of four blessings. The first three blessings are regarded as required by scriptural law:
- The food: A blessing of thanks for the food was traditionally composed by Moses (Berakhot 48b) in gratitude for the manna which the Children of Israel ate in the wilderness during the Exodus from Egypt.
- The land: A blessing of thanks for the Land of Israel, is attributed to Joshua after he led the Jewish people into Israel.
- Jerusalem: Concerns Jerusalem, is ascribed to David, who established it as the capital of Israel and Solomon, who built the Temple in Jerusalem.
- God's goodness: A blessing of thanks for God's goodness, written by Rabban Gamliel in Yavneh. The obligation to recite this blessing is generally regarded as a rabbinic obligation.
Rabbi Abraham Isaac Kook described the order of these four blessings as a “ladder of prayer,” as we raise our sights and aspirations. The first blessing refers to one's personal needs; the second, the physical needs of the nation (through the Land of Israel); the third, the nation's spiritual aspirations (Jerusalem and the Temple); and the fourth blessing, our ultimate aspiration to be a “light unto the nations.”
From My Jewish Learning, "Graced with Food" by Rabbi Jordan Cohen: Moses continues this discourse by telling Israel that the land they are about to enter is, “a land where you may eat food without stint, where you will lack nothing….”
Keep in mind that, despite the miraculous manna that God provided for the Israelites in the wilderness, their biggest complaints were about hunger and the lack of variety in their food. This promise, then, must have been an incredibly attractive temptation for the people. However, Moses reminds them, they must never forget the source of their sustenance. Therefore, they must always remember, after they have eaten their fill, they must offer thanks to God.
Maya: Sometimes it's a struggle for some people to get food to say the blessings because they don't know where their next meal might come from. So when they can get it, they are very grateful. Sometimes people more fortunate take it for granted, so this blessing reminds you that you should be grateful for what you have.
Ben: Where did the grain come from?
Hazzan: It came from God.
Dean: The grain came from seeds that were planted. That came from the ground, which came from the earth, which is in the universe.
Ben: And why are there only five grains.
From Torahmusings.com, Gluten Intolerance, Wheat Allergies and Mitzvos: There are five species of grains that the Torah considers to be the “official” grains for all purposes. These grains are wheat, rye, barley, spelt and oats. While not spelled out in the Torah itself, the Mishna [1] rules that only these five species can be used for making matzah, become chametz, require HaMotzi & Birkas HaMazon, and are subject to the mitzvah of challah and the prohibition of chadash. Other starches or grains, rice included, do not fit into this category. Of significance for our discussion is the fact that four out of these five species contain gluten, with oats being the only one that does not. For the Gluten intolerant or allergic individual, aside from the general difficulties encountered in having a varied and fulfilling diet, the fulfillment of these otherwise rather simple mitzvos can often become quite challenging. ..... for all practical purposes this essay will treat oats as one of the five grains of the Torah without any ambiguity or question
Samantha: It's good for people who are gluten intolerant so that they can still say the birkat hamazon if they want to, but they need to eat oats. It's good that they have an option.