Steve and tony always have pens handy on them wherever they go. This is for two reasons: a), Tony sometimes gets ideas bursting into his head at all hours of the day (including when they’re happily sleeping, much to Steve’s bemusement) and Steve’s skin is very often the closest writing surface he has to jot down his ideas. This tends to end with Steve having to stay very still whilst a rather manic tony scribbles designs and equations onto his back or arms.