I have been trying to root some garlic cloves by suspending them over an aluminium foil dish containing tap water.  After a day or so, pin prick holes surrounded by dull spots appear in the bottom of the dish and the water leaks away.  What's coming out of the garlic and corroding the aluminium?  A control dish containing just water is unaffected.

Foil wrapped chocolate rabbits comprise a highly inflammable aluminium wrapper, surrounding chocolate which melts at slightly above ambient temperatures.  Together the chocolate and the foil completely enclose an air space. 

When heated up, one could imagine either that the enclosed air would expand, leading to an explosion, or the aluminium might catch fire, or the chocolate would melt before either of the other catastrophies. Is it known under what circumstances each process dominates?