When popcorn is heated over a flame, the kernel burst open. why does this occur?

A kernel of popcorn contains water that expands when the kernel is heated, causing it to pop. The equations below give the “popping volume” y (in cubic centimeters per gram) of popcorn with moisture content x (as a percent of the popcorn’s weight).

Hot-air popping: y=-0.761(x-5.52)(x-22.6)
Hot-oil popping: y=-0.652(x-5.35)(x-21.8)

For hot-oil popping, what moisture content maximizes popping volume?
What is the maximum volume?