Answer: There might be milk where the baby is
Si and Am hear the baby crying and go upstairs in the hope of finding milk. As the song 'We Are Siamese' says, "Do you hear what I hear? A baby cry / Where we finding baby, there's milk nearby / If we look in baby buggy, there could be / Plenty milk for you / And also some for me." Lady is furious and barks and growls at them, and chases them away. Unfortunately, when Aunt Sarah comes home and finds the house in a mess, she blames Lady. The Siamese cats pretend Lady attacked them, and Aunt Sarah goes out and buys a muzzle for her. Lady freaks out when the shop assistant puts the muzzle on her, and runs away.