Add "title" to my factors using facet_grid

I would like to add text / title for my factors using ggplot2.

For exemple for data from {reshape2} library:

library(reshape2)
library(ggplot2)
ggplot(tips, aes(x=total_bill, y=tip/total_bill)) + geom_point(shape=1) +
   facet_grid(sex ~ .)

Factors labels are: Female and Male.

How can I add above them the title "sex"?