drupal form process

hi
i created my own content type
i implemented hook_form , hook_validate ,hook_insert and everything work fine ,

but i'm curious about something
hook_validate take $node as parameter , i don't see in which phase (by which function) drupal has created this object
and assign the form variable (such as title , body and other custom fields) to it ,

for example i have a custom field named $form['joke'] in my hook_form()

in the hook_validate($node) i can use $node->joke

i want to know where drupal create this node object .

thanks