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Messages that only the sender and receiver can read. Everyone else gets a big fat nothing.
Your mom texts you: 'I know what you did last summer.' You reply: 'Shut up.' No one else sees it.
Your crush sends you a message: 'I like you.' You reply: 'Ugh, I know.' No one else sees it.
Your teacher sends you a message: 'Don't forget homework.' You reply: 'I won't.' No one else sees it.