Why does Hester tell Pearl she wears the letter?

Pearl asks a lot of questions. Pearl repeatedly asks about what Hester's letter means. Hester is unwilling to unfold the truth to her; the price is too high. Hester replies,

"Silly Pearl,” said she, “what questions are these? There are many things in this world that a child must not ask about. What know I of the minister’s heart? And as for the scarlet letter, I wear it for the sake of its gold thread!”

As Pearl pesters Hester further, Hester says,

“Hold thy tongue, naughty child!” answered her mother, with an asperity that she had never permitted to herself before. “Do not tease me; else I shall shut thee into the dark closet!”