A couple weeks ago, Alexa and Cailin were singing the "if you're happy and you know it" song to each other, and Alexa just decided to try to get Ian to join in on the fun. She said "if you're happy and you know say ba-ba!" And wouldn't you know, he said "ba-ba!" I thought for sure it was a fluke, but then she sang it again, and he complied again. This went on about 6-7 times when I realized I needed to capture this on video. I got my phone out just in time to capture the last few seconds of him feeling cooperative. And get a load of his big finish. It's priceless.
We've tried several times since then to get him to do this again, but either he no longer knows he's happy, or he's just not interested in being his sisters' puppet/parrot.