Popcorn, a variety of Indian corn called Zea
mays everta, is different from ordinary corn in
that it has a harder outer shell. As the corn is
heated, moisture in its starchy grains is
converted to steam, causing a tremendous
pressure to be exerted against the shell. The
tough shell stands up under the pressure for a
while but eventually breaks down and the kernel
explodes with such force that is throws out its
white pulpy insides, making an audible popping
sound in the process.