But it's the pelvis that really tells you about someone's sex. The pelvis is made up of three separate bones. At the bottom of the spine sits the sacrum, a wide, thick bone, shaped like a slice of pie and full of holes. On either side sits the “innominate” or “no-name” bones, two relatively flat, softly curved slabs, each with a socket for one of the hip joints and a notch that allows the sciatic nerve to pass from the spine down into the leg. In females this sciatic notch begins to spread widely as a girl matures, while the front and back of a girl's pelvis becomes wider to accommodate the possible birth of a child. In men, the sciatic notch is narrower, as is the entire pelvis.

Overall, male skeletons tend to be larger and more robust than those of females, but there are exceptions to every rule. We've all met plenty of robust females and small, gracile males.

If sex is tricky to determine, age is really tough. After all, there are only two sexes-but a skeleton might be any age from 0 to 100. I personally have enough trouble telling the age of a living person, even when I can look at indicators like posture, hair color, and wrinkles.

Because exact age is so hard to figure, a forensic report usually gives age as an estimated range-say, thirty to forty in an adult; twelve to fifteen in a teenager or “subadult.” Subadults' ages are easier to estimate, since their bones and teeth mature at a pretty steady and well-documented rate for the first fifteen or sixteen years. Then their bones undergo changes that are a little less predictable as they enter their early twenties. The bones usually don't get any bigger after that, but they do continue to mature until the middle to late twenties.

Age-related changes continue until the day we die, showing up in our ribs, pelvis, and weight-bearing joints-knees, hips, ankles, and spine. The ends of our ribs are stressed every time we take a breath, while the bones of our pelvis grate together throughout our entire lives. The amount of movement is usually so minuscule that we don't even notice it-yet over time that movement is enough to wear down the underlying bone in ways that an anthropologist can use to read a person's age.

Likewise, although the joining, or articulating, bones in the ankles, knees, hips, and spine are covered with a generous cushion of cartilage, sooner or later the cartilage wears down, leaving a record of every day we've stood upright and all the thousands of miles we've walked. And as the cartilage wears down, like the rubber on a tire, the underlying bone begins to show changes: first some irregularities around the edge of the joint, then some roughening of the gliding surfaces. With extreme age-related changes, the whole joint can appear to bubble and boil with bony convolutions, and the weight-bearing surfaces might even collapse.

Age-related changes aren't limited to the legs or spine. The arms, hands, and shoulders are also susceptible to disease and can also show lifelong signs of wear and tear. And then there are the teeth: Are they rugged and unstained? Well worn? Missing, with subsequent absorption of bone?

You'd think estimating a person's stature, or height, would be easiest of all, and you'd be right-if you had a complete body or the right bones. If all you've got is, say, a rib, you're out of luck. Stature can best be calculated from the leg bones, though the arm bones come in a good close second. The length of any of these bones can be entered into a mathematical formula which can then be used to calculate the stature of an individual within a limited range.

Finally, we come to race, a subject that can quickly become touchy and politically charged. For forensic anthropologists, though, race is less of a political topic than a matter of procedure: What can we find out about a mass of bones or body parts that will help police figure out who the person was? In our society, people tend to identify themselves by race, and their friends, family, and coworkers usually know them that way, too. So Dr. Bass made sure we knew the latest thinking on how skeletal structure might vary, depending on a person's racial background.

The bones of a person's mid-face-eye sockets, cheeks, nose, and mouth-reveal our ethnic heritage. For example, Negroid heritage is displayed in a skull with a wide, flattened opening for the nose, wide-set eye sockets, and a forward projecting set of upper and lower jaws. Likewise, a long, narrow nose with a high-pitched nasal bridge and oval-shaped eye orbits tells me that the skull probably belonged to a Caucasian, while in someone of Asian parentage, I would expect to see relatively flat cheekbones and a nose whose characteristics fall somewhere between Negroids and Caucasians-neither flat nor high-bridged.

To learn all of this had taken us months-but by the time Dr. Bass threw his latest challenge at us, we'd all gotten pretty good and he knew it. Yet as he'd given us this week's collection of bones, his words had had the ring of triumph: “In twenty years, no one has ever gotten this one right.” Why not? It seemed to me that every one of us could have measured the bones on the tray, identified the morphological traits, and then told him that we were looking at a tall, well-muscled White man in his sixties. Why would such a simple problem have stumped students for the past two decades?

It was Friday night before I found my way back to the lab where Tyler and I worked together, measuring each bone and documenting our findings. We'd learned through the grapevine that our conclusions didn't differ from anyone else's. Yet I just couldn't shake the feeling that I was missing something.

The night before my report was due, I returned yet again to the lab. I ran my fingers over the contours of the bones and stared at them for hours in the semi-trancelike stillness that was often where I got my best ideas.

And then, suddenly, I knew. I couldn't say how. But I finally understood the secret answer hidden within this apparently simple problem. I hurried home and spent the rest of the night writing my report.

The next day, Dr. Bass collected our reports and began his usual oral questioning. “How many think this skeleton was male?”

Every hand in the room went up.

“Middle-aged?”

Unanimous again.

“About six feet tall, plus or minus an inch or two?”

Yes again.

“White?”

Every hand in the room rose-except mine.

“Miss Craig?” said Dr. Bass. He was always so courteous and formal, I couldn't tell whether he was surprised or not.

“I think the man was Black,” I said into the sudden silence.

Could it be that Dr. Bass was actually at a loss for words? For a moment, he just stared at me. Then he laughed, the way he usually did when one of us got it wrong, and my heart sank. Finally, he sighed.

“Well,” he said, drawing out his words for emphasis. “I never thought I'd see the day.” He shook his head and picked up a photo from his desk-a picture of the person whose bones these were. He was indeed male, middle-aged, tall-and Black. My classmates looked at each other and then at me.

Dr. Bass interrogated me further when we met in his office. “Miss Craig,” he asked me, “how did you know the man was Black?”

I tried to recall exactly what I had seen last night in the lab, which specific detail had triggered my flash of insight.

“It was his knees,” I said finally. “The joint just looks Black.”

Dr. Bass ran his hand across his buzz-cut, looking for all the world like the frazzled D.A. on Perry Mason. “You may somehow have stumbled upon the right answer, Miss Craig,” he said sternly. “But your work will never stand up in court if you can't prove what you know.”