I've just realised why DS is doing this...
for the last few months, whenever Liebling wants to play with Mirror-Liebling, he reaches out to the mirror and then grabs the side and shakes the mirror. He's not shaking it, he's looking behind the mirror, trying to get Mirror-Liebling to come out and play! How clever of him to realise there may be a door or a window in the way! The mirror must feel like a window to him.
