All about keys:
Don't you realise that when you listen to the radio and try to play along the songs on your guitar/piano, the songs are always a little off, in slightly different frequencies? The truth of the matter is, often much of the material is not at 440Hz, defined as 'concert pitch'. Then really what is an A and what is a Bb? In fact, Handel used to tune his A to as low as 420Hz. And really, guitar songs tend to be sharper due to its sounding nicer when it's sharper, while piano songs tend to be relatively flatter, as the piano usually is a little flat due to the tuning mechanism. And really when songs are floating around in your head, there's always a definite key to that song itself. When I'm writing, there's a natural key for a certain song. It's interesting, isn't it. Since I've got bad ears, I'd take out the handphone, find the key using the 'composer', and scribble it down. Haha.
Eve Cassidy's 'Time after time' is a good song.