Why Did I Do That?

I caught my 5-year-old playing with his sister's dollhouse today and didn't say a word. Instead, I sat down next to him, pretending to be completely fascinated by how he was moving the little furniture around. Then, after about five minutes of this odd behavior, I suddenly said, 'Hey kiddo, let’s go build some LEGO castles!' It's not like me to just ignore something when it happens in front of my eyes. So why did I do that? Was it because I secretly enjoy playing with dolls too or was it something else entirely? What would you have done in this situation?