The emphatic instruction to cool completely before assembling pie this is critical to prevent soggy pastry demonstrates understanding of how steam and heat affect pastry structure and why seemingly minor detail determines success or failure of entire dish. Hot filling placed in raw pastry shell generates continuous steam during baking that saturates bottom crust faster than it can cook through and crisp, creating pale soggy layer that never achieves proper texture even with extended baking time. The moisture from hot filling also begins cooking and softening raw pastry before oven heat can set structure, preventing formation of flaky layers as butter melts prematurely without creating steam pockets that separate dough into distinct crispy sheets. The complete cooling requirement rather than just warm or room temperature reflects understanding that even moderately warm filling continues releasing moisture and partial heat affecting pastry negatively, making patience essential virtue in pie assembly that cannot be rushed without consequences. The make-ahead benefit where filling can be prepared up to 3 days in advance transforms critical cooling requirement from inconvenient waiting period into practical planning advantage, allowing flavor development and complete temperature equilibration. The physics involved mirrors other cooking situations where temperature differential matters: adding cold ingredients to hot pan drops temperature preventing proper sear, placing hot food in refrigerator raises internal temperature risking spoilage, pouring hot liquid into cold glass causes thermal shock and cracking, demonstrating that temperature management throughout cooking process affects outcomes beyond just doneness. The soggy bottom problem extends beyond meat pies to fruit pies with juicy fillings, quiches with wet custard, and any pastry-based dish where moisture-rich filling contacts raw dough, requiring strategies like blind baking, using thickeners, coating with chocolate or egg white, or in this case simply cooling filling completely before assembly, proving that understanding moisture and temperature dynamics prevents common failures that ruin otherwise well-executed dishes through single overlooked detail.