I get that baking requires precise temperatures and ingredients, because you’re going for a chemical reaction. But in pie crust, there is no yeast to activate and no eggs to curdle. Just flour, sugar, salt, butter, and ice water. And the butter must be cold, and cold tap water won’t do.
Why though?
Cold butter, as it’s worked into the dough, creates little pockets/layers of butter(rather than being absorbed into the flour) that when the crust is baked, creates a flaky crust rather than a single mass.
Similar to the layering of butter and dough in a croissant.
To add to running coach: If the butter isn’t cold, working it into the dough could melt it. That is not what you want. You want little blobs of intact butter surrounded by flour/water mixture. The ice water further helps the butter stay unmelted.
Oh.
Conversely, if you make pastry with the fat melted into hot water, you get a firm crust that can be carried around, as for picnics, without it collapsing into crumbs.
Likewise, you want to avoid working the dough too hard, both to preserve those little pockets (the heat of your hands can melt the butter), and to avoid over developing the gluten – leads to a tough dough. Alton Brown’s trick to this is to use vodka rather than water in his pie crusts. Ethanol wets the flour but doesn’t allow the flour to form gluten.
-DF
The science is that butter is largely water. When the butter is cold and trapped, you end up with little pockets of water that, when heated, burst into steam and leave air pockets that make the crust light and flaky. The same idea is used in all flaky pastries like biscuits or croissants, and intensified by repeated folding to create layers.