Instructions

1

Understand why the filling is bubbling over. The reason your filling is bubbling over is that there is so much heat and steam building up inside the pie as it bakes, that something has to give. Since the fruit or filling is the softest substance in there, it's going to find a way out.
2

Find a way to vent the steam without ruining the crust.
3

Keep the filling below the crust level. This option though not the best, leaves a little room inside the pie for the steam to raise the filling without going over.
4

Buy a piebird. If you can find one that is. Your grandmother might have one of these. Piebirds are little ceramic birds that you place in the middle of the pie. They are actually shaped and painted like birds with their heads thrown back and beaks open. They have small holes set in their hollow bodies. Set them in the pie with the open beak tip just outside of the top of the crust. The steam will go out through the piebird, leaving the pie perfectly intact.
5

Cut drinking straws. Can't find a piebird? No problem. Just cut up a plastic drinking straw and place them in the pie, preferably through little decorative splits in the crust and they will vent the steam just like the piebirds. Macaroni can also be used in place of the drinking straws. And there you have it, a perfect, non-filling-bubbled-over pie.