Final answer:
Under-steamed milk results in a watery texture that cannot hold latte art designs due to insufficient temperature and steam applied to create the necessary microfoam.
Step-by-step explanation:
When steaming milk, if you have a watery texture that will not hold a latte art design, your milk is under-steamed. Properly steamed milk should have a creamy, velvety texture that is achieved by introducing the right amount of steam to create microfoam within the milk. When milk is under-steamed, the temperature and steam have not been applied sufficiently to create the microfoam necessary for latte art, resulting in a watery texture.