A Shih Tzu (UK: , US: ; Chinese: 西施犬; pinyin: Xī Shī quǎn literally 'Xi Shi dog', also spelled Shittsu) is a toy dog breed, weighing from 4 to 7.25 kilograms (9–16 lb) when fully grown. While the Shih Tzu is an excellent watchdog because of its alert and active nature, it was not specifically bred for this purpose. Unlike the Lhasa Apso, which was bred to be a sentinel dog that enjoys high perches and is wary of strangers, the Shih Tzu prefers to be close to its companions and will often offer strangers its affection. Because of its friendly nature, the Shih Tzu tends to interact well with other dogs and with children and adults. They are generally well behaved and suitable for families. Shih Tzu have a number of respiratory problems related to the shape of their face and head. Brachycephalic airway obstructive syndrome is a common problem and mostly affects dogs with short noses (brachycephalic breeds).