When heated, what is a cause for expansion of 'popcorn' to build sizes 20 to 50 times?

Maize or corn is a cereal grain and a seed. 'Popcorn' is a variety of corn kernel which expands and puffs up when heated and the foodstuff produced from the expansion. A 'Popcorn Kernel's' strong hull contains the seed's hard, starchy cell with 14-20% moisture, which turns to steam as the kernel is heated. The pressure from the steam continues to build until hull ruptures, allowing the kernel to forcefully expand from 20 to 50 times its original size.

Some of the corns are cultivated specifically as popping corns and a special kind of 'flint corn', is the most common of these. The term 'Pop Corn' first appeared in John Russell Bartlett's 1848 dictionary of Americanisms. During World War II, sugar rations diminished candy production and compensated by eating three times popcorn.