No new pictures this week — I spent the whole weekend back in the classroom

One of the ways adulthood totally rules over childhood is that you can pick only the kind of class you’re interested in. If I had any professional, artistic or intellectual ambitions, I’d be up the road at Berkeley taking instruction from some of the finest minds on the Pacific Rim. Don’t fret, I’m having none of that: I parked my fanny in a class on lightweight backpacking taught by a former member of the Iowa Mountaineers (not kidding: it’s a club of flatlanders who travel to mountains to climb them).

The teacher is Steve, the guy who took me snow camping several weeks back. He belongs to a local Sierra Club chapter that conducts lessons every spring instructing people how to stay alive in the forests and mountains for days on end with nothing but the clothes on their back and a pack full of survival essentials strapped onto it.

The most memorable image: Eleven of us sit at tables in a small meeting room in a nondescript office park on the outskirts of Palo Alto. Steve faces us from behind a table containing a blue backpack that looks big enough for moon missions. Steve notes that he routinely toted 60-70 pounds in this leviathan before he saw the light and started trimming his load. While describing his idea of a "lightweight" pack, he opens a zipper and plucks a smaller pack — one that might hold 15 to 20 pounds of gear fully loaded — from the beast’s upper thorax.. Then he starts talking about his "ultralight" pack and snatches a shiny little number from the beast’s lower abdominal cavity.

Clever bit of stagecraft. Steve could take that act to Vegas; I hear there are great trails in the mountains outside of town.

For years, one of the primary appeals of backpacking has been the melding of dreadful suffering with wondrous outdoor vistas. Most folks are content to gawk at mountains and forests from their cars or camper vans, but those who insist on seeing them up close have been forced, until recently, to carry equipment built tough enough for Everest expeditions. A few people who were not Sherpas became indignant that lugging a 50-pound pack up a hillside turned an afternoon amid nature’s wonder into hours of praying for the day’s end or death, whichever came first. They made up their minds to enjoy the show and save their shoulders (and knees, hips, ankles and feet), and the lightweight-backpacking movement was born.

People who are fanatically weight-conscious enough can get their camping kit down to about five or six pounds, not counting food, water or cooking fuel. The point of Steve’s class is not to produce fanatics, but to show perfectly levelheaded outdoorspeople how to get their packs down to the 12-15 pound range.

A couple of my classmates have no choice: one is a woman in her 50s who is recovering from a terrible car crash that left her in a wheelchair for several years. She can walk, slowly, and now that she’s back on her feet, she’s determined to get back in the mountains while she still can. The guy sitting next to her figures he can keep hiking on his bad knee if he cuts enough pack weight. Next to me sits a guy in his early 70s who craves campouts but can’t let a backpack trip up the wires on his pacemaker. His plan: a fanny pack and a satchel slung over one shoulder.

Among hikers, the people who are into lightweight backpacking are considered at best eccentric, at worst crazy as bedbugs. This is odd, given that the very first thing you want to do after putting a heavy backpack on is take it right back off. It’s perfectly rational to wish one’s pack were not so heavy, and yet those who take this idea and run with it are the ones considered loco.

Steve draws a curve on the whiteboard explaining how each extra pound of weight takes a certain number of miles off your hike. The bigger the load, the more fuel needed to move it. And it works in reverse: the less you carry, the less you consume.

The trick is to reduce the three heaviest things in your pack: pack, shelter and sleeping gear. Steve shows how to get each component down to 2 pounds apiece. Trade the tent in for a tarp and a lightweight bug net. Use a down quilt instead of a sleeping bag. Get a frameless pack. Find multiple uses for gear: your rain poncho can also be your tarp. Get a scale and weigh everything; put it on a spreadsheet to see how fast it all adds up.

Gear’s only half the story though. The rest of it is about finding just the right campsite — on soft ground, out of the wind, sheltered from storms — and adapting your hiking style to account for all the extra miles you can cover without the backbreaking weight.

His lecture brings one "wow, I never thought of that" after another. Classmates are amazed to learn about a host of Web sites and backyard entrepreneurs who build highly specialized, super-lightweight gear, often by hand.

Steve’s mantra is that cutting weight always starts upstairs: Something in your head will never be as heavy as something on your back. Which, come to think of it, applies to just about everything.