Pastry rises in the oven primarily because of the formation and expansion of gases inside the dough or batter. The main factors that make pastry rise are:

• Leavening agents: These produce gases such as carbon dioxide either chemically (baking powder, baking soda) or biologically (yeast). The gas forms bubbles within the dough, causing it to expand and rise.
• Heat from the oven: As the pastry heats up, gases trapped in the dough expand and the leavening agents produce more gas. This expansion stretches the dough structure.
• Air incorporated during mixing: Mixing or creaming butter and batter adds air bubbles which also expand upon heating.
• Setting of the dough structure: As the dough heats, its cell walls stretch due to gas pressure and then dry and set, maintaining the risen shape.

Thus, the rising process depends on incorporation of air, generation of gases mainly carbon dioxide, and the stretching and setting of the dough's structure as it bakes. Overmixing or underbaking can cause collapse due to improper gas, air, and structure balance.