Well inside little pieces, or kernals of corn, there is a little droplet of water, and the hard skin around it. When you begin heating up the corn, the water inside also begins to heat and eventually turn into steam. Once this happens, the starchy outside of the corn begins to get soft and then eventually the pressure gets too much and the soft starchy bit pops out, making corn into popcorn!
Popcorn is a special type of corn. What makes it so special? Each grain has a thin, but very tight coat. When you heat the popcorn up, the little bit of water inside it turns to steam. This steam really wants to get out of the tight coat, but it’s trapped there until enough energy is built up for it to explode the whole grain of corn! So in order to pop properly, the corn needs just the right amount of moisture content inside it which will produce just the right amount of steam in the grains for them to pop. Have fun popping!