Not big enough for a conversation. Small enough to just say, to one of the four.
You notice something, and there isn't really a next step for it. Not big enough to mention to anyone in particular. It isn't news, and it doesn't need a reply. It's the kind of observation that gets lost between one thought and the next if there's nowhere to put it.
One of the four gets it. That's the end of it.
Saw something and did a double take. Forgot why you walked into the room, again. That song came on and you had to say something. None of it needs a reply.
There's no minimum length for a text, and no expectation that one message turns into ten. Free for the first seven days, no card, if you want to see whether that's actually how you'd use it.