trigger ContactAfterTrigger on Contact (after insert) {
    List<Contact> consToUpdate = new List<Contact>();

    for(Contact conInTrigger : trigger.new){
        Contact c = new Contact();//Create a new instance of an as-yet-unidentified Contact
        		c.Id = conInTrigger.Id;//Set that Contact's ID to an existing ID from the Contacts in Trigger.new
        		c.Department = 'IT';//Change a field value on that Contact
        consToUpdate.add(c);//Add it to our List so we can update it later when all Contacts in Trigger.new have been prepared
    }
    update consToUpdate;//Perform the DML update call outside of our for-loop to avoid Governor Limits (150 DML per transaction)!
}