Killer in the school

You push open the classroom labeled 4d and are amazed to see a student sitting at one of the desks. You politly ask him how to get into the canteen. "You still don't know what your up against do you" he replies. "He will kill us all and theres nothing you can do about it" You are speechless. "By the way if you want to get into the canteen you must answer my riddles" he continues "if you answer correctly I will give you the key for it, if you give me a wrong answer then your pathetic quest to save this school will be over.Here is the first riddle:
"I have no locks, hinges or doors yet hidden inside me a treasure is held, what am I?" Will you answer: