Build it Solar brings it again! This might be common knowledge but, heat rises and cold falls. This is the action behind thermosiphoning.
The short and sweet of it is that you run some cold water through metal pipes that are being hit by the sun to heat it. You have a circulating system where water from the tank (it’s raised higher than the pipes) falls as it cools, there by flowing to the heating pipes. As this water heats from the sun, it rises to exit the pipes and ends up in the tank again, this time on the top. This leaves you with hotter water on top and colder water on the bottom, circulating and heating more and more.
If that explanation left your head spinning, then the link to this project should help.
I first heard of thermosiphoning thanks to Ampersand Project. I might be lucky enough to go there towards the end of the year and help with a retreat.