Problem: In past times, mending was an art. A meticulous tailor or seamstress could mend rips with thread raveled from hems and seam edges so skillfully that the tear was practically invisible. When the raw material cloth was worth more than labor, it made sense to expend labor in saving it. Today clothing is considered a consumable item. Mass-manufactured clothing is less expensive than the labor required to repair it. Many people buy a new piece of clothing rather than spend time mending. The thrifty still replace zippers and buttons and sew up ripped hems.
What used to be considered an art, back in the day?
The answer is the following: mending

Problem: There is a strong relationship between the properties of wood and the properties of the particular tree that yielded it. The density of wood varies with species. The density of a wood correlates with its strength (mechanical properties). For example, mahogany is a medium-dense hardwood that is excellent for fine furniture crafting, whereas balsa is light, making it useful for model building. One of the densest woods is black ironwood.
What hobby is balsa wood often used for?
The answer is the following: model building

Problem: As economic and demographic methods were applied to the study of history, the trend was increasingly to see the late Middle Ages as a period of recession and crisis. Belgian historian Henri Pirenne continued the subdivision of Early, High, and Late Middle Ages in the years around World War I. Yet it was his Dutch colleague, Johan Huizinga, who was primarily responsible for popularising the pessimistic view of the Late Middle Ages, with his book The Autumn of the Middle Ages (1919). To Huizinga, whose research focused on France and the Low Countries rather than Italy, despair and decline were the main themes, not rebirth.
What methods, applied to the study of history, led to the perception of the Middle Ages as a time of recession and crisis?
The answer is the following: economic and demographic

Problem: The broad field of animal communication encompasses most of the issues in ethology. Animal communication can be defined as any behavior of one animal that affects the current or future behavior of another animal. The study of animal communication, called zoo semiotics (distinguishable from anthroposemiotics, the study of human communication) has played an important part in the development of ethology, sociobiology, and the study of animal cognition. Animal communication, and indeed the understanding of the animal world in general, is a rapidly growing field, and even in the 21st century so far, a great share of prior understanding related to diverse fields such as personal symbolic name use, animal emotions, animal culture and learning, and even sexual conduct, long thought to be well understood, has been revolutionized. A special field of animal communication has been investigated in more detail such as vibrational communication.
What are some fields of knowledge concerning the animal world that have been revolutionizes in the 21st century?
The answer is the following:
animal emotions, animal culture and learning