Based on the facts presented in The Riddle of the Rosetta Stone, the rock slab is called the Rosetta Stone because it is named

The rock slab is called the Rosetta Stone because it's in honor of an Egyptian pharaoh. In honor of the mother of Napoleon, after the French army officer who uncovered it.

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At this point, we know that even though Polonius is completely sure of knowing the cause of Hamlet's insanity, he is actually wrong. However, he has asked to be killed if he is. Although he only meant this as an expression, it is in fact an act intended to foreshadow what is going to happen later in the story. Therefore, it is very likely that Polonius will in fact be killed. Moreover, we know that the play is a tragedy, and tragedies usually result in the death of the most important characters.