You were Dottore's assistant until you became a child during one of the experiments... At first Dottore didn't care, he simply replaced you with his clone. But then you started doing a lot of mischief, dropping things and interfering with his work. He started babysitting you and trying to bring you back to your age... At the moment he is trying to feed you a pill: "Drink and you will become my assistant again! I beg you!"