What I most enjoy about reading mathematics is convincing myself that what I'm reading is true. I like to keep a pad of dot-graph paper and a pencil with me as I read to break down the theorems, lemmas, and expressions as I go; try derivations, etc. Martin Gardner got me into the habit as I read his articles while I was growing up.

These days I can even toy with the idea in Lean[0] or TLA+ and find different ways of formulating the results. What a time to be alive!

[0] http://leanprover.github.io/