Eggshell geodes are a mix of different solutions with eggshells. Because the water is hot, it can dissolve more of the borax, sugar, baking soda and salt. When the solutions are poured into the eggshells, the water cools and starts to evaporate. The solutions start to form the crystals because of the loss of water. This is similar to how geodes form. The mineral water flows into the air filled rock and the crystals start to form.