Besides the flexible resin required, there is something else to consider----the molds. Pump-style shoes are simple to make. They can pop out of the mold easily. Boy shoes cover more of the foot (like the girl's boots do) so they are problematic. When molding, the rubber has to fill the shoe in order for it to remain hollow. This means that the mold can be damaged the first time the original shoe is removed and the risk is greater each time you use it. The mold rubber is soft (unlike the doll's foot) and is more likely to tear when trying to remove a fresh cast. Mattel uses a completely different process for making the shoes (using expensive metal molds instead of the rubber ones).