Dafuq did I just read

Are you over 40? Test your mental agility here:- Read this out loud: This is this cat. This is is cat. This is how cat. This is to cat. This is keep cat. This is an cat. This is old cat. This is person cat. This is busy cat. This is for cat. This is forty cat. This is seconds cat. NOW READ THE THIRD WORD IN EACH SENTENCE.