Final answer:
A 'teacup' Yorkshire terrier is so small it can fit into a teacup, but selective breeding to achieve such small sizes can result in serious health issues for the dog.
Step-by-step explanation:
The tiny type of Yorkshire terrier referred to in the question is known as a "teacup" Yorkie. These miniature dogs are called 'teacup' because they are small enough to fit into a teacup or a person's hand. While they are popular for their size and convenience for urban living, it is important to be aware that this extreme miniaturization can lead to numerous health issues. Some of these issues include collapsing tracheas, digestive problems, heart defects, and other medical challenges. Selective breeding of the smallest dogs can exacerbate these health risks.
Compared to a microscopic house mite, which is about a half a millimeter in size, a teacup Yorkie is much larger but still fits into the category of being a significantly small animal. Nonetheless, despite being bigger than a microscopic organism, the health implications of such small domestic pets should be a serious consideration for potential owners.