What is the sound of a fly smashing on a windshield?

Why are trumpets pitched in Bb? Here's the clearest explanation I've seen so far.

There are three kinds of instruments which are transposing:

1. Instruments where historically players would have been expected to change instruments to play in different keys. This includes trumpets and horns which started as natural instruments (and where changing instruments was typical in the early days of the valve) and also instruments like the clarinet (in orchestras it is assumed that players use B flat and A clarinets).

2. Instruments which are conceived as part of a consort, that is a range of sizes all with the same fingerings pitched in different keys and octaves like saxophones and saxhorns.

3. Auxiliary instruments like English horn, alto flute and so on which use the same fingerings as the "main" instrument.