What is your most useful study aid? Describe it and explain why it is useful in helping you to study?

Cell phone. It is a gift which my father gave me on my 20st birthday. It color is white and it is very modern. Its brand is IP4. The second reason is very useful. I can connect my cell phone to internet to study and chat with friends free. And I can play game, reading newspapers, magazine; watch TV comfortably. Moreover, it helps me to keep contact with friends, family and others. I can give message to them on birthday, New Year, or other special day. I can talk directly without go to anywhere. Or I can express love to other. It is necessary to use cell phone to keep contact. Including, I love my cell phone.