What happend? Exercise 4

Hi I wanna ask for my mistake in the exercise 4. here is my code <?php class Person { public $isAlive = true; public $fristname; public $lastname; public $age; } $teacher = new Person(); $student = new Person(); echo $teacher->isAlive; ?> but still , when I hit Save and Submit, it outputs: Oops, try again. Did you add the '$firstname', '$lastname' and '$age' properties?