How to Compose Math Problems
andreinc.net
andreinc.net
I think this is highly underestimated: there seems to be some sort of a duality between learning & teaching, where you can't really practice one efficiently without the other.
This has been observed in part by guys like Feynman[0]; paraphrasing: "learn a topic by teaching/simulating teaching it to a child", which in addition to the above, demands for the topic to be explained in simple terms.
Although the latter is also highly valuable.