Backpacks are convenient. They can hold your books，your lunch，and achange of clothes，leaving your hands free to do other things.Someday，if youdont mind carrying a heavy load，your backpacks might also power your MP31player，keep your cell phone running，and maybe even light your wayhome.　　The backpacks electricity-creating powers depend onsprings used to hang a cloth pack from its metal frame.The frame sits againstthe wearers back，and the whole pack moves up and down as the person walks.Agear mechanism converts vertical movements of the pack to rotary motions of anelectrical generator，producing up to 7.4 watts.　　The backpack could be especially useful forsoldiers，scientists，mountaineers，and emergency workers who typically carryheavy backpacks.For the rest of us，power-generating backpacks could make itpossible to walk，play video games，watch TV，and listen to music，all at the sametime.Electricity-generating packs arent on the market yet，but if you do get one eventually，just make sure to look both ways before crossing thestreet!　　1. Backpacks are convenient because　　B they can hold as many things as you want to carry.　　D you do not have to carry things withyou.