Talking about useful things, we can mention almost all the things ever, but mine is MP3-player. There are a lot of advantages and disadvantages about that: for example, I'm never alone, I mean if I am upset, I shall just turn my player on and music won't let me be got down. Music is always with me and I never think of what to do - just listen to my favorite songs.
So, the advantages of having a player are: happiness, assurance in myself, etc.
But there also are the disadvantages: player can be out of battery and it needs to be charged, player can distract me from my house- or homework.
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