From Scientific American:

Introduction
Do you remember the last time you baked cookies, bread or cake? Did your baked good turn out perfectly? Or was it a bit too flat or perhaps rubbery and tough, or maybe with clumps of dry ingredients? The problem might have been in how you mixed the dough—or with the type of flour you used. In this science activity you will knead, stretch and punch some pretty remarkable doughs and discover what provides structure and elasticity to your baked goods. Next time you prepare dough for bread, pizza, cookies, cake, pie or any other baked good, you’ll know what to do!

Background
Wheat flours mainly consist of carbohydrates and protein, with some fiber. They are classified according to their gluten (or protein) content for a good reason. Getting the right portion of gluten (the protein that naturally occurs in wheat) is essential to getting the right texture in your baked goods. Wonder why? From the moment you bring a liquid ingredient (such as milk or water) in contact with wheat flour, the individual gluten proteins in the flour unravel and hook onto one another, creating strong bonds. With time, an elaborate network of interconnected gluten strings forms. This network holds the dough together, giving it its structure.

Kneading the dough slowly unfolds the entangled network and aligns the long gluten strings in a stretchy, layered web. A pinch of salt helps as well because it neutralizes electrically charged parts of the gluten, allowing them to better slide along one another. The result is an elastic, stretchable dough that traps gas bubbles. Sometimes a dough can be stretched so thin it becomes translucent, making the network of gluten visible with a magnifying glass or microscope. It is the absence of this intricate gluten network that makes gluten-free baking a challenge.

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