"Imagine you're making a pizza for Barbie, and she likes the super-thin crust."
One of the keys to getting a pizza dough (or a bread dough) that stretches properly and rises well is developing the gluten. It's the elastic gluten network that holds all the lovely gas bubbles that the yeast creates. Once the dough gets in the oven, the gluten needs to be able to stretch even further so those gas-holding pockets can expand. And let's not forget stretching a ball of pizza dough to form it into a disk—no gluten development, no happy stretching.
Most recipes tell you to keep kneading until the dough is smooth and elastic. That's simple enough, right? The dough goes from being a clumpy, lumpy mass to something that is cohesive and, well, smooth. And instead of tearing off in ragged chunks, it begins to stretch. The more you knead, (or the longer it rests, for long-fermented doughs) the stretchier it gets.
But when do you stop kneading? How elastic does it need to be?
Oh, if only there was a simple test for checking the elasticity of the dough!
But wait! There is! It's the Windowpane Test!
Ask about the windowpane test in any serious baking forum and it's almost guaranteed that some wise guy will tell you to throw a hunk of dough at the window, and if it sticks, it's done. In kinder, gentler groups, someone will probably explain the windowpane test.
But pictures are so much better than words, aren't they? To begin the windowpane test, pull off a small bit of dough, about the size of a marble. Flatten it out, then begin gently stretching it.
Keep stretching. Imagine you're making a pizza for Barbie, and she likes the super-thin crust.
If you can keep stretching an pulling the dough until it's a thin membrane that you can see through, the dough is elastic enough. It's ready
Here's a closeup of fingers seen through the dough. You can even see the webby network of gluten.
Keep stretching and even the most elastic dough will eventually tear because it's simply too thin or you've poked a finger through. But a well developed dough should be able to stretch very, very thin before it breaks.
Doughs that are made with gritty flours (like whole wheat, for example) will tear sooner, simply because the bits tear through the membrane. This is the same thing that happens in the bread as it bakes, and it's one reason why breads made from whole grain flours tend not to rise as well as those made from more refined flours. But even with those doughs, you should be able to stretch it well enough to see that thin membrane.
If your dough passes the windowpane test, you know that gluten is developed well enough for it to hold the bubbles while it rises, and stretch even more as the gasses expand from the heat. Simple, hmmmm?