It has been a week for obsessions, from Twitter to a new addiction, Plants vs. Zombies. The first was the subject of a two-day conference where I was a panelist, even as social media played a role in the Iranian election unrest. The other is the latest computer game that has consumed too much time of the 9-year-old and, um, others, in our household. It is amusing and addictive. If you play it you will never look at mushrooms or vegetable gardening in quite the same way again.