How should we teach the mechanism of osmosis?

What is the cause of osmosis? Typically, with my students of 14-16 years old (GCSE courses), I don't delve into the mechanism. It's the pattern that matters most at this stage. But as students progress to my IB biology course (16-18 years old) and explanations lean more on molecular details, the mechanism becomes more important.

In conjunction with the bigger pattern (i.e. water potential) what should we teach as the mechanistic cause?

🚨 Update: I've moved to a new home! You can read the newest, updated version of this article on my new website here: https://christianmooreanderson.com/how-should-we-teach-the-cause-of-osmosis/