If you have read receipts enabled but want to read the occasional text without letting the sender know you’ve seen it, just use 3D Touch to peek into the Message. (Note that this tip requires an iPhone 6s or 6s Plus.)
To do this, firmly press on the Message preview. Lift your finger after you’ve read it and the message will still be marked as unread on the sender's end. If you decide you want to officially read it after all, press harder to pop into the message.
Next, learn how to edit a text message, mark an iMessage as unread in the Messages app, and how to see if someone read your text.
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