"flute quartet in d-major, kv 285, II. adagio" (mozart)
This was Einstein's favourite flute quartet and so, if you buy into "the Mozart effect," then you might want to start listening to it too.
Mozart composed this work on December 25th, 1777, at the age of 21. He had composed it, along with two other quartets, in order to make 200 gulden (old German / Dutch gold coin) and extend his stay in Mannheim. As with most Mozart stories, there is also a passionate, but short-lived, romance.
At under 3 minutes, it's a short movement, but one with an elegant, simple melody that will make sure it stays in your head for quite a bit longer. Its ethereal quality, gentleness and delicate pizzicati, makes it a longing serenade... perhaps one sung in secret.