The only difference between a Pokémon and an egg is that an egg has a non-zero number of steps left to hatch. This number is kept in the Pokémon's variable called eggsteps. For actual Pokémon, this number is zero. The method egg? checks whether an entity is an egg; it returns TRUE if it is, and FALSE if it is a Pokémon. It is used as follows: $Trainer.party[0].egg? This looks at the first entity in the player's party, and checks whether it is an egg or not.