I was one of these fortunate children referred to in GUEST Cathy's post who were taught tonic solfa, and it did indeed become second nature.

To this day I can take any melody, major or minor, and sing it using the tonic solfa notes, without having to stop and think, it is automatic.

If this method were still taught today, we would have far less requests on forums and YouTube for chords/tabs, since a side benefit of learning solfa is that you can immediately identify the harmonies as well.