Hi Charlotte, the idea is to settle the weight of the dough on the rim in a stable manner before crimping, which will help prevent movement of the dough. Since a triangle/pyramid is a very stable shape, it will help keep the dough from slipping down or slumping during baking. This is easy to accomplish by pinching the excess dough on top of the rim with your thumb and index fingers so it comes to a point, with the weight of the dough balanced evenly on the rim. You can then crimp or twist the dough, as desired, to provide a decorative finish. 

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