Stopping a bain marie or steamer from boiling dry

 
Jar lids - who'd have thought?
 

A bain marie is a great way of gently cooking heat sensitive foods, such as chocolate, and steaming preserves many of the nutrients that are lost through other methods of cooking.

Putting a jam jar lid into the pan with the water will act as an "alarm" that will rattle to warn you if the pan is in danger of boiling dry.  

Simple, but effective.