After nearly 6 months of one way play, our son is finally joining in the fun.  Oh sure, in the past he would grab, chew, and throw his toys, laugh at his toys, and even laugh in response to our antics, but we still consider this one way play.  After all, we would be practically standing on our heads to elicit said laughter, while Mr. Calvin was simply a laughing lump of baby.  Don't get us wrong, those days clearly beat out the days when he didn't even acknowledge our existence (a trait that is sure to return in about 15 years), but they don't hold a candle to the new and improved Calvin, now with real interactive mode so that he will actually take part in the play, rather than just laugh while watching.  Go ahead...laugh while you're watching!