Why are pie shells pricked thoroughly with a fork before baking?

Pricking pie shells with a fork before baking is done for two key reasons:

1. To prevent the pastry from puffing up: When the pastry cooks, the trapped air inside expands and can cause the shell to bubble up unevenly or even crack. Pricking the bottom and sides allows the steam to escape, resulting in a flat, even crust.

2. To prevent the bottom from getting soggy: The steam from the filling can make the pastry dough soggy if it's not able to escape. The holes created by the fork allow the moisture to evaporate, leaving you with a crispy, flaky crust.

Pricking the pie shell is particularly important for recipes with wet fillings like fruit pies or custards. It's also useful for blind-baking, where the shell is baked before adding the filling.

However, it's important to note that you don't need to prick shells for every type of pie. For example, if you're using a pie crust recipe that includes a lot of butter, you might not need to prick the shell at all.