Death ray not fully functional

Spidermonkey now has his own userpic.  Since I still haven't had time to upload pictures of his artwork for the school production of Hair, here's what he told me about his day yesterday.

"M [spider's dangerously bright friend] built a death ray at break time, but it didn't work." 

Apparently, if you rub the bottom of a Coke can with chocolate (which is - again apparently - a weak alkali) for three hours, you get a concave shiny surface capable of focusing the sun's rays into a deadly beam.

The break-time death ray was scuppered by insufficient chocolate-rubbing time and the sun going behind a cloud, but M allegedly built a working one at home. 
"How did he know it worked?" I asked nervously. 
"He tried it on a leaf."

Has anyone heard of this before?  Is it based on any kind of reality at all?