What feature introduced in Pokémon Gold and Silver lets compatible Pokémon produce Eggs?

Pokémon breeding is the feature introduced in Pokémon Gold and Silver that lets compatible Pokémon produce Eggs.

In these games, players can leave suitable Pokémon at the Pokémon Day Care. If the Pokémon share a compatible breeding group, the Day-Care worker may discover an Egg, which can eventually hatch into a Pokémon.

The offspring generally belongs to the mother's species, while certain moves can be inherited from the father. Ditto provides a special exception because it can breed with almost every Pokémon that is not in the No Eggs Discovered group.

Breeding was one of several major additions in Gold and Silver, alongside a real-time clock, held items, the Steel and Dark types, and the split of the old Special statistic into Special Attack and Special Defense. It should not be confused with Pokémon evolution: breeding produces a new Egg, while evolution changes an existing Pokémon.