To add to your comment, when you do get a question that the junior dev has been sitting on, it's a good indication for you that this area of the system needs be

To add to your comment, when you do get a question that the junior dev has been sitting on, it's a good indication for you that this area of the system needs better documentation. If following the 20 minute rule, when you do get a question, it might be worth spending 20 minutes to answer it (and provide appropriate background) as well.