WHY DOES popcorn pop and not ordinary corn? Because popcorn has a thicker hull and contains starch with superior puffing quality, say Brazilian scientists who compared four varieties of corn and three kinds of popcorn (Nature, Vol 362, No 6419). They found the popcorn kernel's ability to absorb heat efficiently was also a critical popping factor.
While the popcorn kernel absorbs more heat much faster than corn, its hull stays cooler. The lower temperature at the hull minimises the chances of its burning when the starch is just hot enough to puff up. On the other hand, the corn hull becomes dark and calcified when it reaches the temperature where starch expands the most, thus preventing the corn from popping.