Many years ago, before mobile internet was reasonable and before wireless internet was available, and before even electrical outlets were something which could be counted on to be present on trains, I took a 6 hour train ride. I had no laptop. I printed out, on paper, the entire source code of the project I was working on, and brought a red pen. I read through the whole thing, from start to finish. Many subtle bug fixes, refactorings, and efficiency improvements were made that day.