Freeze factor: Professor Popsicle knows how to survive a plunge into icy waters. Do you?

CRACK! The ice beneath your skates splinters. In a split second, a gaping hole opens and you drop into frigid waters. You panic. You gasp. What next?

That's the question Gordon Giesbrecht (GEEZ-brekt), aphysiologist (scientist who studies the body's vital functions) and director of the University of Manitoba's Laboratory for Exercise and Environmental Medicine, tries to answer. Giesbrecht is one of the world's leading experts in hypothermia: a condition in which the body's normal core temperature drops from 37[degrees]C (98.6[degrees]F) to 35[degrees]C (95[degrees]F) or below. Unlike researchers who work in the comfort of labs to study the human body's response to cold, Giesbrecht seeks firsthand experience. That's why he regularly hops into lakes--and even truck-size tanks of ice--as trained assistants monitor his body's core temperature, pulse, and other vital stats.

Giesbrecht's toe-numbing experiments have earned him the nickname "Professor Popsicle"--a title that has rightfully stuck like a wet tongue on a cold metal pole. "I probably hold the world record for the number of times I've experienced hypothermia," he boasts. That record: 37 occasions.

Why would anyone voluntarily turn into a human ice pop? Giesbrecht is gathering data on how humans handle a spill into cold water--usually defined as water that's 20[degrees]C (68[degrees]F) or below. He hopes his findings will stomp out common myths about hypothermia. "Most people have a poor idea of what happens when you fall into cold water. They think you'll die of hypothermia in 5 to 10 minutes," Giesbrecht says. The truth is, "you won't die for at least an hour or two." That means you might be alive when rescue finally arrives.

The trick: Learn how your body regulates its temperature, and know how much time you have before your nerves (bundles of cells that carry signals between parts of the body and the brain and spinal cord), muscles, and heart shut down due to the cold.

FIGHT THE FREEZE

Your body constantly works to warm you. In a process called metabolism, billions of cells release energy stored in food. Your body converts this fuel into heat, which blood vessels ferry to every inch of your body.

Meanwhile, an internal thermostat located in the hypothalamus tracks your body's temperature. This peanut-size gland in the brain controls your body's core temperature--keeping it at a steady 37[degrees]C (98.6[degrees]F)--no matter what outside temperatures you face.

Sensors along your skin and inside your body feed the hypothalamus information about the temperatures of each part of your body. "The hypothalamus takes that information and sums it up," says Giesbrecht. If the hypothalamus concludes that you're cold, it instructs your body to start a process called peripheral vasoconstriction (puh-RIF-uh-ruhl VASE-oh-con-STRIK-shun): "The blood vessels in your fingers, hands, and feet, for instance, will constrict. The diameter will decrease and the blood flow will decrease," he says. The reduced blood flow keeps body heat from escaping through your extremities.

If that's not enough to keep you toasty, you'll start to shiver (experience rapid cycling between relaxed and contracted muscles). "We shiver to produce heat," Giesbrecht explains. Shivering can increase your heat production by five to six times the normal amount.

REMEMBER: 1 MINUTE, ID MINUTES, 1 HOUR

Too much time in icy water can be a challenge for your hypothalamus. If you were to accidentally fall into cold water, you would lose a lot of body heat to the surrounding water. That's because heat energy moves from warmer objects (you) to objects with less heat (the cold water). And due to its greater density, water pulls heat away from the body about 27 times faster than air at the same temperature. This rapid heat loss, says Giesbrecht, would eventually bring a victim into the first stages of hypothermia (see Nuts & Bolts, right). And he should know. Last winter, Giesbrecht plunged into a vat of ice water on the Late Show with David Letterman to demonstrate what people should do if they find themselves in similarly cold waters. His advice? Remember this frosty phrase: 1 minute, 10 minutes, 1 hour.

After taking the icy plunge before 5 million late-night viewers, Giesbrecht began to explain: "You have one minute to get your breathing under control." That's important because your body's initial cold-shock response causes uncontrolled breathing that can make you gasp, possibly leading you to swallow water and drown.

Next, you have about 10 minutes of meaningful movement to try to escape the ice-cubelike conditions. Soon after that, your muscles and nerves will lose their might, and you'll be saying farewell to your strength. So make the most of this time. "When you're freezing, it's hard to lift yourself straight up," Giesbrecht says. "Instead, put your arms on the edge of the ice and do a flutter kick so your body becomes horizontal with the water's surface. Then kick and pull." That way, you're dragging yourself onto the ice--an easier task than pushing up onto it.