Usually acquired in the morning, you hear a piece of music or a song - often an irritating piece that you wouldn't listen to by choice.
This tune will then repeat in your head unabated whenever it can.
This is how you kill the ear worm:
Sing - in your head - this version of God Save the Queen:
The ear worm will now be gone.
Don't worry, the worm will not be replaced by God Save the Queen.
[May not work outside of UK]