What's the difference between a man and a woman?
The question seems too silly to be science: its answer is so obvious that every stand-up comic has a different way of saying it. (Even the gender jokes break down by gender: male comics say women go to the bathroom in packs. Female comics say men wouldn't understand what they talk about in there anyway.)
But the difference is only obvious most of the time.
In some unusual cases, resulting from sex-change operations or medical conditions, the usual indicators of male and female can contradict each other in the same body. The best-known recent example is South African runner Caster Semenya, who has been put through "gender verification" amid suspicion over her muscular physique and low voice.
But the same confusion has cropped up in legal battles over who can be married to whom, and when the "M" on a driver's licence can be changed to "F". It can also intrude into the lives of ordinary people, when medical diagnoses reveal that their hormones, chromosomes or anatomy don't sit entirely on one side of the line.
These cases have left judges, doctors and athletics officials - those tasked with drawing a line between the sexes - struggling to find a reliable gender test, some trait that divides all men from all women.
But scientists say they don't have one yet.
"I think most people think of it in binary terms; that is, you're either one or the other," - male or female - said Myron Genel, an emeritus professor of paediatrics at Yale University. "In reality, it's more of a continuum."
Researchers say it is difficult to know how common cases of conflicting gender indicators are: some people are reluctant to make their conditions public, and others may live and die not knowing they have them.
Scientists estimate one in every 100 people has some kind of "disorder of sex development", where one indicator of sex is subtly or severely out of step with the others.
The signposts of a person's sex include the chromosomes, X and Y and others, that are the blueprints for sexual development. Hormones such as testosterone and oestrogen are the chemical messages. There is sexual anatomy, built on those chemical orders.
And there is a psychological sense of identity, which some scientists refer to as "gender", as opposed to "sex", which is everything physical.
But the signs don't always point in the same direction.
The reasons can include "androgen insensitivity syndrome", whereby people born with XY chromosomes, generally considered the combination for maleness, do not respond to testosterone.
In cases of "complete insensitivity", which occur once every 20,000 births, according to the National Institutes of Health, the chemical signal to turn male is missed. These children are born looking like girls and grow up as women, and may not discover their condition until adulthood.
Other conditions can set women's hormones and genetics at odds in the opposite way, making them appear unusually masculine despite their female XX chromosomes. And still others can create confusing markers of sex in men, leaving them with male anatomy and hormone levels but two or more X chromosomes.
In other cases, the cause of the disagreement is a sex-change procedure; in these circumstances, anatomy and chromosomes would no longer agree. The National Centre for Transgender Equality estimates that 0.25% to 1% of the US population has changed gender, or intends to.
Together, cases such as these have led some researchers to believe it is impossible to find a universal boundary between male and female.
"There's not one sign or unique parameter . . . that clearly defines sex - as in clearly separates, unequivocally, males from females," said Eric Vilain, a professor of human genetics at UCLA.
But, in athletics and the law, which often insist a person be considered either male or female, authorities are still looking for a line.
In sports, the first gender test was the simplest: in the 1960s, women athletes in some events had to parade nude in front of judges, who were looking for male anatomy. A man had, apparently, competed as "Dora" for Germany in the 1936 Olympic high jump (and finished fourth in the women's event).
In the late 1960s, this was replaced by a less humiliating test of genetic samples. But this process wasn't perfect: it "failed" female athletes with androgen insensitivity syndrome, since their genes made them seem male.
Now, most international sports groups have stopped requiring all female athletes to undergo gender testing. (Male athletes never had to.) The logic is that regular drug testing, which requires officials to watch as the athletes urinate, will still screen out masquerading males.
But the rules still allow competitors or judges to request one of these tests on a female athlete. That's what happened to Semenya (18), who won the 800m race at the August world championships in Berlin and whose case reveals that sports officials are still struggling with how to tell that a woman is a woman.
Reports have said Semenya has been found to have testes inside her body, calling her a hermaphrodite - a term scientists in the field say is outdated. But even that may not disqualify her: the track association's rules permit some such athletes to compete as females.
Watching this process, some experts on sexual development have said the track association's standards are too murky, and that it should pick one trait as the be-all, end-all marker of sex. The international track federation said recently it planned to hold a symposium soon to discuss changes to its gender-testing policy.
Alice Dreger, a professor at Northwestern University, said officials might decide based on an athlete's upbringing: if she was brought up a girl, she should be counted as a woman. Or they could set a threshold for male hormones in an athlete's blood - too much, and she would run with the men.
"To me, it's no different than deciding where the foul line is," Dreger said. "The line is not drawn by nature, it's a line we draw on nature."
Doctors say they often struggle to break the news - to tell a woman who has come in for infertility treatment, for instance, that her chromosomes are XY.
Some patients ask, "Am I a man?" Vilain, the California genetics professor, said. "You send the question back, you know: Are you really a man? Do you have any doubts about it? And often the answer is no. So why would it change?"
Cynthia Johnson (44) grew up in California knowing she was different: because of androgen insensitivity, she was born with XY chromosomes and what doctors term "ambiguous" genitals, which were surgically altered to make them appear more female.
About six years ago, Johnson said she started learning more about her condition, and now says she's comfortable with the idea that she is a woman whose body contains a contradiction.
"It's something different," she said. "But I think everybody's different."
