A blockhead thought experiment is a way to think about how your decisions affect other people. It's like imagining that everyone around you has a big block of wood instead of a head, and if you make a bad decision that affects other people, it's like you're hitting them on the head with the block. So with every decision you make, you should think about how it would affect other people around you. You don't want to be the one hitting them on the head, so you should always think of the consequences of your decisions.