Minor Second

One semitone. The ratio is sixteen to fifteen, which is complicated enough that the two notes never settle into each other. It is also the distance from the leading tone to the tonic, which is why the most uncomfortable interval in music is also the one that makes an ending feel inevitable.

A grinding, beating quality when the notes are played together in the middle of the keyboard. Played one after the other it is simply the smallest possible step, and most people find it easy.

Widen the interval one semitone at a time here and hear the roughness go. The dissonance is about how close the two frequencies are, not about the notes themselves.

As the leading tone resolving to the tonic, which is the strongest pull in tonal music. Inside maj7 chords, where the root and the seventh are a semitone apart and the chord still sounds soft because the notes are spread out. In film scoring, wherever unease has to arrive without a single loud sound.

With the major second, which is one semitone wider and sounds ordinary rather than painful. Played melodically the two are easy to mix up, so drill them together.