Like any good teacher, Stephen Hawking knows he has to grab his students' attention and hold on tight.
That isn't easy when your field is theoretical physics. It's even harder when you are paralyzed and must use a machine to talk.
But Hawking succeeds, with the aplomb of a seasoned comedian who knows how to work an audience.
For example, while kicking around the concept of black holes--collapsed stars that snare everything, even light, with their powerful gravity--Hawking tells you what it's like if you're unlucky enough to fall into one.
"I'm sorry to disappoint galactic tourists, but flying into a black hole would be like going over Niagara in a barrel.
"It might be useful for getting rid of garbage, and some friends, but any astronaut venturing into a black hole would get torn apart and turned into spaghetti."
The setting was a cavernous lecture hall at Northeastern University packed with about 800 people for a free public speech on subjects ranging from black holes to wormholes in space.
In his motorized wheelchair, under hot floodlights, Hawking looked tiny and helpless sitting in the middle of the huge stage. As the lights dimmed, Hawking, unmoving, began the recorded lecture in his synthesized, mechanical voice.
Hawking, a British-born Cambridge University professor and author of the bestseller "A Brief History of Time," has been called the most important theoretical physicist since Albert Einstein.
He pioneered work on black holes and the Big Bang theory of the origin of the universe. In the process he helped legitimize cosmology, the study of the foundations of the cosmos.
His theories are as thorny and dense as theories come, but Hawking brings them down to earth, explaining the cosmic with the comic.
In some ways, Hawking is straight from an academic ivory tower. He wears a slightly frumpy brown tweed suit and thick glasses. His hair is a ragged mop.
But at age 48 he is hopelessly paralyzed, the victim of a 25-year battle with amyotrophic lateral sclerosis, or Lou Gehrig's disease.
His clothes hang loosely on his withered frame. He seems pinned by gravity to the red leather seat of his wheelchair. His arms and legs are twisted.
Perhaps most trying, Hawking must use a computerized voice synthesizer just to speak.
Ask him a question and painful moments pass as Hawking, using his crippled right hand to manipulate a computer cursor, composes an answer letter by letter, word by word.
In his public speech at Northeastern, Hawking warmed up the audience with a few deft quips and one-liners.
He remarked that black holes were first discussed at Cambridge. "I mean the real Cambridge," he added, to the delight of the Boston audience.
Discussing his book, Hawking told the audience, "I knew it was going to be a success when it was translated into Serbo-Croatian."
Hawking also reserved time for several more personal gatherings just with students. In one small meeting room, on a sunny afternoon, physics students clad in jeans and sweat shirts waited patiently as Hawking was wheeled into the room for a question-and-answer session.
Nervously at first, then gathering courage, the students stood to ply Hawking with their queries. Once a question was asked, the room was silent as Hawking took eight, 10 or even 15 minutes to compose his answers.
The answers, the students said, were worth waiting for.
Hawking was born in Oxford, England, on the 300th anniversary of the death of Galileo. "About 200,000 other babies were also born that day," he said. "I don't know whether any of them was later interested in astronomy."
The son of a Scottish mother and a Yorkshire doctor, Hawking grew up near London. As a boy, he seemed so average academically that a friend bet a bag of sweets that he would never amount to anything.
"I don't know if this bet was ever settled, and if so, which way it was decided," Hawking said.
At Oxford, Hawking studied physics and then began doctoral work in cosmology at Cambridge. But in his early 20s he was diagnosed with ALS, and the disease progressed swiftly.
Doctors said the affliction would eventually kill him, and Hawking became depressed. "There did not seem much point in working at my research, because I didn't expect to live long enough to finish my Ph.D.," he said.
Then, he became engaged to Jane Wilde, who later became his wife.
"This gave me something to live for," Hawking said. "If we were to get married, I had to get a job. And to get a job, I had to finish my Ph.D."
Hawking finished his degree, the disease stabilized, and he and his wife eventually had three children. Today he is Lucasian Professor of Mathematics at Cambridge, a position once held by Isaac Newton.
Hawking visited Northeastern last year for a scientific conference. During his stay he kept up a rigorous schedule, attending conferences, giving lectures and exploring the city. He insisted on eating lunch with students every day in the cafeteria.
"One night he went to dinner after holding a press conference and attending a Monet exhibit," said Brigid Mast, a physics student at Northeastern who helped coordinate his visit. "After dinner he wanted to wander around Harvard yard. He was exhausting his nurses."
And whenever possible, he taught.
One student asked Hawking about his childhood interest in science fiction.
"Science fiction is written and read by people who would like to understand the universe, and maybe, to control it," Hawking said.
"Now that I have learned to do real science, I don't need science fiction. However, some people would say that my work is all science fiction."
Of course, not all the secrets of the universe can be explained with quips and one-liners. Hawking held technical symposiums that went over the heads of more than a few graduate physics students.
"The stuff he's doing is way up there; it's kind of extraterrestrial," said Robert Sneddon, a graduate physics student at Northeastern. "The kind of math he's using you don't learn in grad school until the fourth or fifth year.
"But he's hilarious. I laughed my head off," Sneddon added. "It shows how brilliant he is, that he can make his work interesting for people who aren't so brilliant."
In one lecture, Hawking discussed the part of his work in which he postulates that black holes can unite and form "baby universes," tiny outcroppings of our own universe.
Particles pulled into a black hole will vanish, travel through the baby universe and re-emerge somewhere else in the universe, he explained.
Such particles would travel in what Hawking calls "imaginary time," which he says exists at right angles to our own time.
Hawking was getting technical. Doctoral candidates were looking dazed.
Sensing he was losing his audience, Hawking introduced a particle of levity.
"The universe seems much simpler in imaginary time. It doesn't have singularities, and it doesn't have a beginning and an end," he said.
"So if you take a positive view, as I do, you could say that it is really imaginary time that is real, and ordinary time that is a figment of our imagination."
The students perked up. They laughed.
In real time, he added, anyone who fell into a black hole would come to a sticky end. "So the motto of anyone who falls into a black hole must be--think imaginary."
More laughter. Hawking had grabbed them again.