How do you rate half a star?

All I see in the review page is the ability to click from one to five entire stars. I don't see how you add a half star. But I see half stars on others' reviews. What's the trick?

If you want to rate something 4½ stars, click 5 stars and click it again. A third click deletes the stars. For 3½ stars, click 4 stars twice, etc.

Thanks. I was wondering how to do this.

Authors rate a half star by publishing terrible writings. I'm just saying...