AngularFireDatabase is not a observable?? code example

Example: AngularFireDatabase is not a observable??

// change to valueChanges()
constructor(afDb: AngularFireDatabase) {
  afDb.list('items').subscribe(console.log);
}
//to
constructor(afDb: AngularFireDatabase) {
  afDb.list<Item>('items').valueChanges().subscribe(console.log);
}