Yes, it is that time of the year again, Paul Nelson Day. PZ suggests we should all make it a point to ask people “How do you know that?” today, and the ones who actually can explain themselves competently will be complimented by being told that they’re no Paul Nelson. A further tradition is that you can invent a new concept (such as “ontogenic depth”) and promise to explain it later. The wonderful thing is that “later” never appears to come. So, what concept do you want to invent today?