Sentence:  Rachel discovered that she could also keep things in her <WSD>backpack</WSD> that were important to her, nobody would know about them because they would be hidden. These important things included a small round rock that she had found ... 
Meaning of the word marked with <WSD> tags:  A knapsack, sometimes mounted on a light frame, but always supported by straps, worn on a person's back for the purpose of carrying things, especially when hiking, or on a student's back when carrying books.