Whenever I read a corollary, even a lemma, I try to prove it myself first before reading the textbook's proof. This is because I usually find the level of difficulty of the proof comparable to the exercises in the textbook, and thus the textbook's proof is its (correct) solution (which is a real bonus since textbooks almost never show its solution to its exercises). With theorems, sometimes they can be proved by me too, sometimes they are too hard to prove, but I always try to think of a proof first before reading its proof. Do you guys do the same thing? Or do you read theorems and their proofs back to back?