Along the same lines: in a math textbook or math-heavy paper, try to prove the Lemmas or theorems yourself, when it’s sensible to attempt it. Probably more suitable in a textbook where they build up to a larger theorem starting from smaller lemmas — try to prove them yourself. Even if you don’t prove them the first time, on a re-read you can try to prove it yourself, as a way to test your understanding, and of course this also deepens your understanding.