When Juliet tells Romeo's "happy" dagger that "this" is its sheath, what is she referring to as "this"?

Read this excerpt from through the looking-glass by lewis carroll. "you might make a joke on that,” said the little voice close to her ear: "something about ‘you would if you could,’ you know.” "don't tease so,” said alice, looking about in vain to see where the voice came from; "if you're so anxious to have a joke made, why don't you make one yourself? ” the little voice sighed deeply: it was very unhappy, evidently, and alice would have said something pitying to comfort it, "if it would only sigh like other people! ” she thought. but this was such a wonderfully small sigh, that she wouldn't have heard it at all, if it hadn't come quite close to her ear. the consequence of this was that it tickled her ear very much, and quite took off her thoughts from the unhappiness of the poor little creature. what question should a reader ask to clarify what is happening in the story? who or what is the voice speaking to alice? when will alice attempt to share a joke? what secret will alice share with the new character? why do whispers tickle the listener?