Most pumpkin pies—like the ones on your Thanksgiving table every year—have fillings that contain pumpkin puree, cream, sugar, spices, and eggs. Because of the eggs, which make the filling nice and thick, these pies need to be baked, often for a long time. In our pumpkin pie, however, we don’t use eggs. Instead, we use gelatin—the same ingredient that is in Jell-O! And instead of baking our pie, after heating up the gelatin and the pumpkin in the microwave, we chill it. (Well, we still bake the crust by blind-baking the crust by itself; then we chill the baked crust and its nonbaked filling.) How does that work, you ask? We use gelatin in the filling. Gelatin is a kind of protein. It’s made up of long, thin molecules. When gelatin is mixed with a hot liquid, its molecules are loose and flexible, and they move around a lot—the liquid …