Anthony (4) has learned to send a reply text message saying, "Ok." A surprising number of my texts could be answered this way without being inappropriate. It's a little disconcerting, because it means that Anthony can now answer texts from my phone in a way that will make the other person think I have some information or agreed to do something without me ever knowing about it. There are also situations where "Ok" could make sense but would seem rude or dismissive; "Congratulations!" or "I'm so sorry to hear that" might be the expected responses to certain messages.
As an amusing exercise, imagine what the effect would be if you answered every text message with, "Ok." Would it work or would people suspect a 4-year-old stole your phone?