Cyndi Lauper inhales helium before she performs live.

The "Girls Just Wanna Have Fun" singer, famed for high-pitched voice, was spotted sucking the voice-distorting gas out of balloons back stage at the Hetrick-Martin Institute benefit gala, in New York, last week.

A source told the New York Daily News newspaper, "Cyndi sucked the gas out of two helium balloons before going on stage to belt out 'Time After Time' and some of her other hits."

The 54-year-old singer also managed to upset her teenage back up singers at the charity concert - which raised money for the Harvey Milk High School for Lesbian, Gay, Bisexual and Transgender (LGBT) youths - when she introduced them to the audience as her "transgender choir".

One of the young singers fumed, "She keeps calling us transgender! We're not f***ing trannies!"

The Scissor Sisters also performed at the Institute's gala dinner.