Water goes first
Starch water poured into a hot pan starts to boil. The steam cooks the food under the lid. The starch stays in the pan.
Starch stays behind
As water leaves, the starch grains swell and then dry out. They join into one thin sheet. The sheet looks like lace because bubbles leave holes.
Oil finishes the crunch
The dry sheet now fries in the oil. It browns and turns brittle. A thin sheet shatters. A thick one stays chewy.
Shunta says:
Try it once with plain water and once with flour water. Put the two pans side by side and tap the edges.
