Starch Gelatinization
Starch gelatinization is what happens when dry starch grains soak up water and heat, swelling into a soft gel.
Think of it this way
Dry popcorn kernels sit loose in a bowl. Add water and heat, and they soak up the liquid and puff into soft, swollen lumps. The grains go from hard and separate to a thick, clumped gel that fills the bowl.
Explaining starch gelatinization by grade level
Think about dry pasta. It starts hard and stiff. When you put it in hot water, the pasta soaks up water and turns soft. The tiny starch bits inside puff up like small sponges. That change from hard to soft is starch turning into gel.