This thevenin theorem is really giving me a headache, and I've really searched many sources in the internet but they confused me more and more. Take this as an [example](http://www.solved-problems.com/circuits/electrical-circuits-problems/1016/thevenins-theorem-two-independent-sources/):
![thevenin][1]

The guy in the above link says:

>The 6Ω resistor is short circuited and the 5Ω one is open. Therefore, their currents are zero and RTh=2Ω .

But see this [link](http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thevenin.html) for example and especially this picture:
![thevenin 2][2]

According to the above the $R_1$ and $R_3$ resistors should be shorted and the only one that should count is $R_2$.


And see this link [also](http://www.solved-problems.com/circuits/electrical-circuits-problems/1005/thevenins-theorem-circuit-independent-source/)

the three links solve the thevenin theorem in different ways and especially in finding $R_{th}$ can somebody explain this to me in a simple way? 

  [1]: https://i.sstatic.net/elWHK.png
  [2]: https://i.sstatic.net/utu5N.gif