A story is told about King Jannai. Elsewhere in the Talmud (Kiddusin 66a) we learn that King Jannai had all the rabbis assassinated, possibly because of an incident (told in Sanhedrin 19a) in which he was humiliated before the rabbinic court. One who escaped the carnage was Simeon b. Shetah – the Queen’s brother.
In our story, the king and queen are having a meal together, and the king laments that since he has killed all the rabbis, there is no one to say grace for them! (The very definition of chutzpah: the man who murders his parents and then pleads for mercy from the court because he is an orphan).
The Queen invites her brother, after making her husband swear he will be safe. Simeon b. Shetah is brought to the meal, but is not invited to eat. Afterwards he is asked to say grace and is given a cup of wine to say it on. The blessing he gives is a mocking one: “Blessed is He of whose sustenance Jannai and his companions have eaten.” He then drinks the wine. They give him another cup and he says grace over that one.
Thus one must have actually eaten something of a meal before saying grace.