The first thing you hear is a soft, descending octave in the first violins, the simplest of musical ideas, and a stepwise progression up the first four steps of the A major scale – along with a little chromatic agitation - over some serenely, almost ecclesiastically sonorous polyphony in the lower strings. This theme is then repeated, loudly, and with the addition of a canon: two beats later, the violas and cellos have the same theme as the violins, but at the third and fifth of the chord rather than the tonic - which means there's a greater harmonic and contrapuntal richness than when we first heard the melody. In the space of 30 seconds or so, Mozart has used an enormous arsenal of sophisticated compositional techniques to create a miniature symphonic drama.
And that's just the first theme. What's wonderful about this symphony is how much Mozart is clearly enjoying himself, in the extra melody he composes at the end of the first section of the first movement, a joyous little tune that symbolises the sheer invention of this symphony; in the contrapuntal conversation between the violas and the cellos and basses in the movement's central section; and in the cheekily inventive coda, with its chromaticism and, again, its counterpoint, this time in an outrageously fulsome four parts.