What one ingredient is added to sweet dough used for tart shells that makes it more sturdy than both flaky and mealy pie dough?

The addition of eggs to sweet dough is what provides the extra sturdiness compared to both flaky and mealy pie dough. Eggs contribute proteins that coagulate when baked, which lends structure and stability to the dough. This is particularly important for tart shells, as they need to hold their shape while supporting fillings that may be more liquid or dense.

In contrast, flaky and mealy pie doughs are typically made with fat and flour, which create a tender, crumbly texture but may not have the structural integrity required for tart shells without the support that eggs provide. The combination of moisture and binding elements from the eggs helps prevent the tart crust from crumbling or becoming overly delicate, allowing it to maintain its form and enhance the overall texture of the tart.