The performance begins with Bea Sullivan-Knoff emerging on stage with a brown paper bag over her head. It says, on all four sides, "Touch Me."

Sullivan-Knoff, a 26-year-old transgender performance artist in Chicago, appears fully nude, inviting the audience to objectify her, she said. It's supposed to be ironic, a literal depiction of the way she and other transgender people feel when the government dictates the terms of their bodies: deeply vulnerable, sometimes terrified.

"There was something empowering about being able to do that and being fine at the end of it," she told The Washington Post.

But in 2016, a venue where she was scheduled to perform called with disappointing news, Sullivan-Knoff said. She could neither perform fully nude nor, as Sullivan-Knoff proposed alternatively, only from the waist-up, because a Chicago ordinance prohibited it.

So Sullivan-Knoff agreed to change her performance - and then she sued the City of Chicago in federal court, arguing that its ban on the exposure of "female breasts" in establishments that serve liquor is unconstitutional.

Sullivan-Knoff's case rests on two main points, one traditional and one relatively novel.

She argues, as does the "Free the Nipple" movement, that laws barring women but not men from exposing their chests are unconstitutionally discriminatory and based on little more than society's sexualization of women. But she also questions the very definition of "female breasts" in the context of transgender people and whether authorities can fairly apply laws based on binary gender to gender-non-conforming people. That problem, she suggests, extends beyond ordinances censoring breast exposure.

For example, she questioned, would police enforce the ordinance against a transgender woman who is legally female but whose breasts are biologically male? What about against a transgender man who is legally male but whose breasts have yet to be surgically reduced? Are those breasts still "female"in the eyes of authorities?

In a ruling earlier this month, a federal judge in Chicago appeared receptive to considering the questions Sullivan-Knoff raised - or at least curious about them.

"At minimum," wrote U.S. District Judge Andrea R. Wood in the Nov. 12 ruling, "these questions pose serious concerns in the abstract, as nothing in the text of the Ordinance provides a clear answer."

In the ruling denying the city's motion to dismiss, Wood agreed that Sullivan-Knoff put forth compelling arguments. She was unpersuaded by the city's defense that female breasts are treated differently than male breasts because they "create a sexual environment." Chicago's attorneys argued that the purpose of the law is to "combat the secondary effects resulting from the combination of nude and semi-nude dancing and alcohol consumption," but Wood called that into question.

She allowed Sullivan-Knoff's case to move forward on her claims that the ordinance violates the Constitution's Equal Protection guarantee, meaning it's discriminatory against women, and that as applied to transgender people it's unconstitutionally vague.

"Stating 'female' and assuming we can all understand whether that applies to us doesn't work when you start to look at trans and intersex folks," Sullivan-Knoff said. "You can find so many different kinds of bodies tied to those two gender markers. The diversity gets lost when you just say it's either an 'F' or an 'M.'"

Sullivan-Knoff's questions aren't just hypothetical. For example, in 2010, police in Rehoboth Beach, Delaware, told WBOC News that they weren't exactly sure how to apply a law banning female breast exposure to a group of transgender women, some who had not fully transitioned, who were sunbathing topless. They ultimately decided not to enforce it, because they said the women still had male genitalia.

Nan Hunter, a law professor at Georgetown University and scholar at UCLA's Williams Institute on sexual orientation and gender identity law, said that Sullivan-Knoff's case reflects the complications transgender people encounter when trying to comply with laws written for a binary gender world. Hurdles arise for those seeking to change their gender markers on driver's licenses or passports, Hunter said, or for those seeking redress for discrimination based on gender identity, which can be complicated because many laws are restricted to sexual orientation or sex.

When it comes to laws policing bodies, she said, gender fluidity can "create a situation where you can have an anomaly," in which the traditional binary gender notions don't always apply to gender-non-conforming individuals.

"The trans movement is forcing institutions across society to question the extent to which gender categories are necessary or even useful," Hunter said. Sullivan-Knoff's case, she said, is "one of those fascinating cases where the law itself may be minor, but it ends up raising really important questions.

In court, the City of Chicago argued that displaying bare breasts "almost invariably conveys sexual overtones" and cited a U.S. Court of Appeals for the 2nd Circuit ruling that upheld a ban on female toplessness in 1998.

But Wood was skeptical of that reasoning, noting that "the heightened sexual nature of female breasts might just be a product of society's sexual objectification of women." She cited a federal ruling last year that struck down an ordinance in Fort Collins, Colo. U.S. District Judge R. Brooke Jackson agreed with plaintiffs, the "Free the Nipple" campaign, that a law banning women from exposing their chests in public "perpetuates a stereotype engrained in our society that female breasts are primarily objects of sexual desire whereas male breasts are not."

Mary Grieb, Sullivan-Knoff's attorney, argued that Chicago's ordinance "blatantly" advances the same stereotype Jackson ruled against in Colorado, a decision that remains on appeal. Grieb may face an uphill battle: In a 2-1 ruling in 2017, the U.S. Court of Appeals for the 7th Circuit upheld Chicago's public indecency law banning exposure of female breasts, an ordinance similar to but separate from the one Sullivan-Knoff is challenging. Writing for the majority, Judge Diane Sykes said the government had a legitimate objective to uphold "traditional moral norms and public order."

But Grieb said Sullivan-Knoff's First Amendment protections as a performance artist are likely to bolster her case.

"The city is relying on societal conventions because that has worked in the past and has been enough to justify these types of laws," Grieb said. "But as society has evolved, the law is slowly but surely, I hope, evolving to catch up."