Ah, before I read the article, from the image I thought maybe it was bottles on both sides, and using the sun to evaporate the water in the lower bottle until i

Ah, before I read the article, from the image I thought maybe it was bottles on both sides, and using the sun to evaporate the water in the lower bottle until it reaches the higher bottle, causing it to tip. Thought that was pretty clever! But probably not accurate. Would be beautiful to design a closed system that works well like that however.