When heat is applied to the top of a stack of pennies, why does the bottom penny melt first? I just watched this video where a blow torch is used on the top of a stack of pennies. 


I'd like to know why the bottom penny melted first. 
 A: I would suggest that the bottom penny does not have the same composition. A zinc penny is mentioned in the comments (BTW, what a tedious video) and zinc has a much lower melting point than copper (420 C vs 1085C). Copper plated zinc pennies were introduced in 1982. Given that it started to melt almost straight away, I think that is your answer.
At about 10 mins onwards in the video you can clearly see from the brightness of the top penny that it appears to be much hotter than those below. Heat conduction can only follow the temperature gradient, so even if the bottom penny was on an insulating surface it couldn't actually get hotter than the penny above it.
EDIT: Something similar has gone on in the UK and is equally illegal). The two-pence coin weighs 7g and was, pre-1992, made of bronze (about 95% copper) and was replaced with copper-plated steel. At peak copper prices, the pre-92 coin was worth significantly more than its face value as scrap metal (I just checked, you could still get 0.3p per gramme). You could also sort out a pile of two pence pieces into pre- and post-92 coins with a magnet - a nice magnetism demonstration, for which I used to have a set of coins from 1975-2010 in a row that you could pass a Neodymium magnet over.
