The thing that made the things for which there is no known maker.
Every painting needs to have had a painter, and every building needs to have had a builder. Every thing for which there is no known maker, needs to have been made by something. Maybe, thousands of years ago, a conversation like this took place... and brought theism into the world. I wonder if the dogmatism came along as quickly as in THIS example?
This is definitely one for the atheists and anti-theists. I don't expect religious folk to really get this one.