We’ve all experienced the pain vicariously—watching a 60-pound fourth-grader struggling to haul 20 pounds of books in a “backpack” that has all the body-hugging fit of a bowling ball. Just imagine toting a full third of your body mass on your next ski tour—hung by two skinny straps cutting into your shoulders.

Skinning up towards the snow-cone summit ahead, I am working extra hard to keep up with my faster friends. The skies are blue, but it's been a slog and the temperature is down right cold. I'm so thirsty, but stopping to pull the water bottle from the bottom of my pack will only set me back farther. By the time I near the top, I'm moving slow and haven't peed for hours.