What does a Trainer do exactly?

What does a Trainer do exactly?

Whether offered as a challenge, or a genuine inquiry, this questions can be answered in so many different ways.  Yesterday, while in a cafe, the view inspired one possible explanation...

"Trainer's open windows so people can get a clearer view of the horizon."

(I'm sure there's plenty of follow up questions to this too, such as; "Why is the window so unclear to start with?" or "Whose responsibility is it to clean the window?" And for those who think more logically, 'Why are heck are we talking about windows?")