where to put my active record callback objects

I'm begining whith rails.
In the rails directory structure, where is the nicest directory tu put
my active record callback objects ?
app/helpers ?
app/models ?
lib ?

May looks a stupid questions but i can't find answers googling.

I put my observer classes in app/models.


I keep my observers in app/observers. Feels cleaner to me.

For it to work, add that directory to load path in environment.rb:

  config.load_paths << "#{RAILS_ROOT}/app/observers"

Thanks for your replies.
I undestand that there is not a rails defined directory to put the
callbacks/observers objects.
As my callback only depends on the model stack, i put it in a new
directory : app/models/callbacks.
Works fine and seems nice for me.