Can you sing in your head?

I was reading a book about the psychology of music by Victoria Williamson called 'You Are The Music'.

I enjoyed the book, it was interesting and informative. I was puzzled by one comment the author made though. She said that when she used to work in a gift shop she would sing along to Eva Cassidy songs during quiet periods. But she did this, in her head. Does that strike anyone else as odd?

I had a go at singing along in my head to Tom Waits. I don't think I can do it. In general I mean, not just to Tom Waits. If you think you can do this, what is it like? If you can't or think, like me, that there's something odd about the idea, can you articulate what that is?