A well-timed line of upside-down text is a low-effort, harmless way to confuse a friend for a few seconds — the fun is entirely in the reaction, not in anything mean-spirited, so it works best on someone who'll find it funny rather than annoying.
Send a normal message, then follow it with a fully flipped one and nothing else — no explanation. Most people's first instinct is to assume their phone glitched or auto-corrected into nonsense, which buys a solid ten seconds of confusion before they realize it's on purpose.
Start a conversation completely normally, then have your replies get progressively more flipped word by word until the whole message is upside down, as if something in the chat is slowly "breaking." It reads like an ongoing glitch rather than a one-off joke, which tends to get a bigger reaction than a single flipped message.
If a friend texts something mundane, replying in full flipped text — as though it's the most normal thing in the world — plays on the mismatch between the tone of the question and the strangeness of the answer. It lands best when you don't acknowledge it's flipped at all and just let them ask "what is this."
This works as a prank because it's obviously harmless and takes seconds to undo — just ask them to copy your message into any flipped-text tool to read it "the right way." Skip it with anyone who might genuinely worry something's wrong with their phone or account.