A longtime friend of Britney Spears has revealed the pop star's penchant for leaving her underwear at home started off as a joke - but now she loves going out without her knickers. The pop superstar's exposed genitals have been snapped several times as Spears clambers in and out of cars during nights on the town - and Britney still thinks her naked ambition is a giggle. Jewellery designer Tuesday Knight, a longtime pal of Spears', admits she's one of many friends urging the wild mom to cover up - but Britney insists she doesn't get the joke she's playing. Knight tells In Touch Weekly magazine, "The underwear thing is ridiculous - and I've told her, `You cannot walk into a store wearing just a shirt with your underwear off.' I've told her that no one will take her seriously when she does this. "I've heard Paris Hilton, Lindsay Lohan and Britney talk about how they shock Hollywood just for the fun of it. They were discussing how they could play a little joke on all of us." But while Hilton and Lohan have given up their vagina-flashing antics, thanks to sobering spells in prison and rehab, Spears has become quite fond of letting it all hang out. Knight adds, "I've watched her spiral downward.... Britney doesn't need to do it and she knows it's stupid, but she says that she just doesn't like to wear underwear."