Create an experiment to study the effects of caffeine. Have people drink water or coffee and have them solve a simple paper maze with a pencil. They solve the maze faster with caffeine. In this situation it helps them solve the problem faster. But that isn't caffeine's only possible effect is it?
Give someone caffeine and put them with someone they are attracted to. What effect? How often do we ask someone out for a coffee as a first date?
Now, do the tests together, operate the maze together with the person you are attracted to. How do those things affect one another? Is caffeine involved?
Now, take them out of a controlled room and put them in one of various physical properties and hierarchies. How does caffeine affect them related to each of those things?
Make one person a boss. Then, imagine that boss has a partner at home they love but they are attracted to their subordinate. Now replace the maze with a creative project where they have to come up with ideas.
Now add the personal history of the two, they had a drunken one night stand. They have known each other a long time and are actually very compatible and would have been a better match in the end. They would have been married instead of having an affair. The subordinate just made a pot of coffee.
What is the effect of caffeine on productivity?