Reply to a message
Long-press the message you want to respond to.
Tap Reply.
A preview of the original message appears above the input box.
Type your reply and send it.
Your sent message shows a reference to the original, so the other person can see exactly which message you're replying to.
Note: Replies only reference one message, they don't create a separate thread. If you want to cancel a reply before sending, tap the X on the reply preview.
FAQ
What happens if I reply to a message that's later deleted?
Your reply stays in the chat, but the reference will show as unavailable.