Most butter contains about 15 to 17 percent water. When the pie dough goes into the oven, the water turns to steam, which is what helps create layers in your dough. In other words, it is the sheets of butter that make your pie dough flaky. If your butter is somewhat warm, then you end up with something that is more like cookie dough than flaky pie dough. Not the end of the world by any means—and a tender, crumbly pie dough is still a good dough—but for a pie crust that flakes and shatters and impresses with its many layers, keep your butter cold, cold, cold.

When do you want your ingredients to be at room temperature? There are two good examples of this. The first is when you are combining sugar and butter for a cake or cookie. If you look at sugar under a microscope you see why they are called sugar crystals. They have jagged edges, and when you mix sugar into room temperature butter, these edges act as an army of little workers with shovels carving out miniscule air pockets within the butter. If your butter is too cold, the sugar—try as it might—can't dig its way through the hard chilled butter; if the butter is too warm, the sugar merely sloshes around, not really being effective at all.

If it is at room temperature, however, that sugar can work its magic and aerate the butter. The act of combining butter and sugar together in this way is actually called "creaming" because when done properly the butter turns light and white like cream. Once you've created a multitude of air pockets, the baking powder or soda you add to the cake/cookie later on expands these air pockets and you end up with a light, tender, fluffy pastry. And all because you started with room temperature butter!

A second example of when ingredients should be at room temp is when you add a liquid such as milk or buttermilk or eggs into a cookie or cake batter. Imagine the butter and sugar you've just creamed together: an aerated fluffy room-temperature glorious mass. The next step in the recipe calls for adding eggs or other liquid to the butter-sugar. If your eggs/liquid are cold and not at the same temperature as what you are about to mix them into, the butter will immediately harden into little cold pellets. And when you bake your cake/cookie you'll end up with lots of little holey pockets from the butter bits. Not good! To keep your crumb even and soft, make sure your ingredients are at the same temperature when combining them, ensuring seamless emulsification.

Moving on to warmer temperatures, there's a valuable baking technique called tempering that will prevent you from making scrambled eggs when you are trying to make a pastry cream or ice cream base. First you start with something like milk or cream that you heat up in a saucepan until it just about comes to a boil. In a separate bowl you mix eggs and sugar. Now you want to combine the hot liquid with the egg-sugar mixture. Do you just mix the two together? Noooooooooo. That's like jumping into the pool, cannonball style. In pastry you want to dip your toe in, one toe at a time, and slowly ease your way in.