Braces

Barb was supposed to wear the braces the dentist gave her for a few weeks to reform her bite before she got new implants.  One day the dentist phoned to see how things were going and Barb was talking to her on the phone.  The dentist said she was talking really well with the braces on.  The thing was Barb didn't have her braces in.  She didn't tell the dentist that though.  When Barb does have her braces in, she whistles her 's'.