Original: 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 .
Simplified: A backpack ( also called rucksack , knapsack , packsack , pack , Haversack , or Bergen ) is a cloth sack put on somebody 's back .