What is the meaning of "va'Yagres"?
Rav Sadya Gaon: He set [my teeth] on edge.
Rashi: He broke. This is like "Garsah Nafshi" (Tehilim 119:20), "Geres Karmel" (Vayikra 2:14).
Palgei Mayim: At the time of sin, it was like eating flour with pebbles inside that break teeth,
What is "Chatzatz"?
Rashi (from Eichah Rabah 1:22): It is gravel (pebbles in earth). The exiles kneaded their doughs in holes that they made in the ground, and the gravel entered them 1 . Hashem told Yechezkel "Ase Lecha Klei Golah" (12:3), to drink in it and knead a small cake in it, so others would see and do so - "v'Hayah Yechezkel Lachem l'Mofes" (ibid., 24:24). They mocked him and did not do so. In the end, their teeth broke.
Ri Kara: It is sand (Rid - pebbles), like "v'Achar Yimalei Fihu Chatzatz" (Mishlei 20:17).
Ri Kara: When they ate the bread, sand in the earth broke their teeth.
What is "Hichpishani [b'Efer]"?
Rav Sadya Gaon: He buried me.
Rashi: It is like a Kli inverted on its mouth. 'Pishon the camel-driver measured with a Kefushah Kli' (Yevamos 107b).
Ibn Ezra #1: He repulsed me (filled my mouth with ashes).
Ibn Ezra #2: This is like "Ki Efer ka'Lechem Achalti" (Tehilim 102:10).
Ri Kara: They lied on earth and ashes.
Rid: He sullied.
R. Avigdor (from Ta'anis 26b): This is like it was taught, on Erev Tish'ah b'Av, one may not eat meat and drink wine. Rav would dip a slice of bread in ashes and eat it, and say 'this is the meal of Tish'ah b'Av', to fulfill our verse.