A backpack (also called rucksack, knapsack, packsack, pack, bookbag or Bergen) is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders, but there can be exceptions. Light weight types of backpacks are sometimes worn on only one shoulder strap, or in the hand like an ordinary bag.

Backpacks are often preferred to handbags for carrying heavy loads, because of the limited capacity to carry heavy weights for long periods of time in the hands. Large backpacks, used to carry loads over 10 kg (22 lbs), usually offload the largest part (up to about 90%) of their weight onto padded hip belts, leaving the shoulder straps mainly for stabilising the load. This improves the potential to carry heavy loads, as the hips are stronger than the shoulders, and also increases agility and balance, since the load rides nearer the person's own center of mass.

In ancient times, the backpack was used as a means to carry the hunter's larger game and other types of prey as a way of easier transport. In the cases of larger hunts, the hunters would dismember their prey and distribute the pieces of the animal around, each one packing the meat into many wrappings and then into bags which they placed on to their backs. The bag itself was made up of different animal hide and skin (depending on what sorts of animals were in the area) and sewn together by the intestines of said animals, which were woven together tightly to make a sturdy thread-like material.

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• Energy-Harvesting Backpack Straps
  physorg.com
  A news description of the Piezoelectric backpack straps that harvest energy from a wearer's movement to power/charge electronics.
• Lightweight Adventure Backpack
  rodneyslab.tripod.com
  Instructions from Rodney Liwanag for making a lightweight backpack. Includes photos and specifications.