Whisk the lemon Jell-O into the boiling water until completely dissolved.
Whisk in the ice cold water.
Whisk in half of the Cool Whip until smooth.
Whisk in the remaining Cool Whip until creamy with no lumps.
Refrigerate the filling for 30 minutes.
Give it one more gentle whisk, then pour into the graham cracker crust.
Refrigerate for at least 6 hours (overnight is even better!).
