title: The role of the model in MVVM
body: <p>I've read a few articles regarding the role of the (Data)Model in the MVVM pattern.
  However, i still could not figure what goes into the model.</p>
  
  <p>Should the model implement INotifyPropertyChanged? If so, what is difference between the VM and the model?</p>
  
  <p>If not, a change in the Model will not notify the VM and the View that it occured. So - considering the logic goes into the Model, it seems obvious that it should notify the ViewModel about some of it's changes. However, isn't it weird to implement INotifyPropertyChanged in both classes?</p>
  
  <p>Thank you very much!</p>