Karl is a double amputee, but not by accident, birth or disease. He is an amputee by choice.
Six years ago, Karl (who asked that his real name not be used) sat alone in a parked car with 100 pounds of dry ice and an obsession to destroy his legs.
"The first thing I did was I used a wooden flour scoop to scoop some granulated dry ice into the bucket. ... It filled the wastebasket with carbon dioxide gas, which was 79 degrees below zero," he said.
Over the next 45 minutes, Karl put his legs in the wastebasket and then kept adding dry ice until it got to the top. "I spent the next six hours well-packed in the dry ice, and then I'd add more dry ice to keep it topped off," he said.
A chemistry major in college, Karl had done his research well.
"I'd done all the thermodynamic calculations, the mass of tissue, how much heat you had to subtract from that tissue to achieve freezing temperatures," he said. "And I knew that after six hours I had certainly achieved more than enough to freeze the full thickness."
After those six hours, Karl calmly drove himself to an emergency room, using the automatic hand controls he had installed in the car.
Within days, his legs began to blacken as the frozen tissue died away, and within a month surgeons had no choice but to amputate both of Karl's legs.
Karl is not a one-of-a-kind medical mystery, however. There are others like him, who believe their bodies don't match the picture of themselves they have in their minds.
"I wasn't born in the correct body," said Lilly, who has twice tried to amputate her legs. "The mind doesn't connect up to the body at all."
Dr. Michael First, a psychiatrist at Columbia University in New York, is one of the few researchers to study patients with this strange obsession to lose one or more of their limbs. The rare condition is called body integrity identity disorder, or BIID.
"When these people see an amputee, they see ... a person of strength being able to overcome hardship, someone to be admired," First said.
Aside from this obsession, First said his BIID patients can appear to be mentally healthy.
"The most striking thing about these people, is that if you were to meet one, you wouldn't have a clue that there was anything unusual about them," he said.
The fascination with being handicapped nearly always begins in childhood, said First. Karl said the first inkling that he wanted to be an amputee came when he was just 5 or 6 years old.
He often took refuge from his own family life in the home of a neighbor who was disabled by polio. Then he saw a young amputee.
"It was kind of like the proverbial light bulb going over the cartoon character's head," Karl said.
His first reaction, he said, was envy. But still, Karl said he considers himself normal.
Dan (who asked that his last name not be used) is a biomedical engineer who lives and works in a small town near the French Alps, and he has carried the same obsession as Karl's throughout his life.
Dan, who is intensely physical and loves hiking and skiing, said he had thought of amputating his own leg using dry ice and a power saw. To reduce his anxiety, he sometimes pretends to be an amputee.
Even when he is exercising, Dan said, "I find myself imagining, OK, how hard would it be, wearing a prosthesis?"