"The greatest sin" joke

Three nuns on a train had been getting to know one
another and decided to tell each other what their
greatest sins were.
The first nun says, "My greatest sin is sex. Every year
I go out for a week and work as a prostitute. Of course,
I put all the money I earn into the poor box."
The second nun says, "My greatest sin is drinking. Every
year I take the money from the poor box and go out
drinking for a solid week."
The third just sits there quietly.
So the first nun says to her, "Come on, we've told you
our worst sins. Now you have to tell us yours."
The third nun says, "My greatest sin is that I gossip,
and I can't wait to get off this train."