Which Pokémon gameplay mechanic did Pokémon Gold and Silver introduce for raising eggs?

Pokémon Gold and Silver introduced Pokémon breeding as a gameplay mechanic for raising eggs.

In the second-generation games, compatible Pokémon left at a Pokémon Daycare can produce an Egg. The Egg usually hatches into a young Pokémon species related to the female parent, while some moves can be inherited from the parents.

Breeding added a new layer to team building beyond catching wild Pokémon. Players could pursue particular moves, abilities available in later generations, or preferred traits by producing and hatching multiple Eggs.

The mechanic is not unlimited: Pokémon are assigned to breeding groups, and many Legendary and Mythical Pokémon cannot breed. Ditto is a special partner because it can breed with most Pokémon outside the No Eggs Discovered group. Breeding is therefore broader than simply placing any two Pokémon together.