Boiling water with hot rocks.

[Circuits]

Read the link, I just skimmed over it but I believe you have to use certain types of rock...


i.e. limestone won't really work, while something hard like granite would be great....

Well, you don't "HAVE" to use certain types of rock - but some are better suited than others.

Rocks which crack can potentially be dangerous to the handler, but will still get the water to boil.

From the article, granite is one of the worst - has fine cracks/pores which cause cracking from steam expansion. The best of the ones tested was basalt.