The relationship between cats and mice could be best described as 'Predator-Prey'.
The egg shell would be most representative of the Earth's Mantle or "Crust", the yolk and white would represent the molten metal inside the earths core I assume.
If you talk about the process of fission in the nuclear field then the most used is U-235 because is the easiest to use. Have in mind that the majority of uses of uranium is U-238 but the easiest to be used is U-235