Complex sentence: Rice Krispies are made of crisped rice (rice grains that are cooked, dried and toasted), and expand forming very thin and hollowed out walls that are crunchy and crispy.
Split into simpler sentences: Rice Krispies are made of crisped rice (rice grains that are cooked, dried and toasted). These grains expand forming very thin and hollowed out walls that are crunchy and crispy.