Well, after you or your tutor put in a good deal of time getting it into tune, it really fries your brain when you take it up again and its out of tune!
Well, there are reasons for that:
1. The guitar is made up of metal parts. Anyone who has read physics 101, will surely know that metals tend to expand and contract with the change in temperature. Its the same phenomenon that bends long pieces of railway track in the heat or bends the pieces of metal in a thermostat. The strings and the supporting do just that. They expand when heated and thus getting tuned down and vice versa. It happens between night and day and when taking the guitar in and out of a cold room.
2. You may have noticed that one particular string, that you just replaced, doesn't want to stay in tune for the first couple of days. This is because the string thins out a bit and elongates while bearing the tension just placed on it. It only does so for a few millimeters and so the delinquency stops faster than it started.
3. The six little turn keys are not so sweet as they appear to be. They also loosen up and start relaxing the strings leading to the guitar losing its tune.
4. the payers also play their part. As we keep striking and pressing the strings, we firstly heat them up, secondly stretch them (specially during bends) and thirdly, when we aren't playing, we keep the guitar in odd positions leading to the keys getting turned slightly - all to only one plausible effect.
So, I guess I just bored you a bit with the why-is-it-so stuff when all you wanted to do was tune it. Well, just go here to tune up.
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