Human interaction - teaches human interaction.

Many people consider this activity and skill to be valuable in themselves, but rationalists have a particular reason for pursuing this. This is because it is easier to notice that someone else is confused than to notice it about oneself. And the analysis of other people's confusion involves skills which transfer directly to the analysis of your own confusion.

The most important skill learned here is the identification of hidden, unshared assumptions. If you and your interlocutor each think that the other is being illogical, then the most likely explanation is not a lapse in logic, but rather an assumption that one of you is making and the other is not. Teasing out that assumption and shining a bright light on it is a necessary rationalist skill, even if you do all of your important thinking alone.