Birkat HaMazon D'Rabanan: Rabbinic Source

Although the mitzva d'oraita is to thank God only when you are satiated from the meal, chazal decreed that we say birkat ha'mazon even when eating only the volume of a k'zayit and even if not satiated. So you must still say birkat ha'mazon after eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread within four minutes--even if you are not full.  This is known as birkat ha'mazon d'rabanan.
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I just read this halacha, Birkat HaMazon D'Rabanan: Rabbinic Source , at www.practicalhalacha.com. I think you will find it very interesting.