Now this is a fun idea:

The high-quality black thong has a panel of flexible material attached to the front that allows chalk messages to be written with the red chalk pen provided. The chalk can be easily washed off and a new message written whenever desired. Humour and wit are restricted only by imagination. (Source)

What would you write on your thong? One's imagination runs wild at these possibilities. I also can't help but wonder if the chalk would rub off on the inside of your pants, which would 1. create a mess, and 2. spoil your message.  I suppose if these don't work out, we could always resort of sticking a post-it note to our knickers.