So I had kids work on adding and subtracting a bunch of numbers in matrix problems in precalculus. And I had some “nice” numbers and some less than nice numbers. And what I loved was that one of my groups was not punching anything into their calculators. (I have this personal aversion to mental math so I tend to use my calculator a lot when I’m at home alone working on problems.) I asked them why and they said they liked to train their brains by doing things sans calculator. Love that!