If you had access to a time-machine and could transfer one piece of knowledge to an influential ancient (i.e. Plato), what would you tell him?
How to make a movable-type printing press. They'll figure out pasteurization and the scientific method on their own eventually, but without a press, they'll lose knowledge almost as fast as they gain it. And as an added bonus, it introduces the concept of mass production.
How to make a movable-type printing press.
This requires a lot of work. I'm not sure that they had the metallurgy to do this. The antikythera mechanism suggests that the answer is yes. But the printing press as a whole requires a lot of different technologies to come to together. The screw press, without which moveable type is highly inefficient, was not around until around 100 CE or slightly earlier(I'm under the impression that late medieval versions were generally better and more efficient than Roman era screw presses but don't have a citation for th...
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