Excellent question! First of all, congratulations on getting your egg "safely" to the ground. The theory behind this experiment is to build up energy into the system by raising it to a large elevation, giving it gravitational potential energy. When you release the box, the potential energy is transferred over to kinetic energy, the energy of motion. When the box impacts, all that energy has to be transferred, distributed, dissipated in a relatively short time, which can result in some of it being transferred to the egg, which breaks. The dent in the side of your box bears testament to the fact it absorbed some of that energy, energy that the egg would have borne had it not had the container around it. I recently did a similar egg drop, but I used a parachute (plastic grocery bag) and a conical design, like an ice cream cone. The parachute spread the energy out over a longer time frame, slowed the rate of descent, which gave the system less energy to be dissipated. We had cotton balls and inflated balloons inside the cone capsule to give an "air-bag" energy absorption quality. We were able to get ours to survive a four-story fall as well!