91. Read the following passage, then answer the question below.

Few animals have a reputation as impressive as the tardigrade, a microscopic creature, less than a millimeter long, that lives in moss, soil, and the sediments of lakes. Tardigrades are nicknamed water bears because of their plump bodies and clumsy, eight-legged walk. Their fame, however, comes from what they can survive. When their surroundings dry out, tardigrades curl up, expel almost all of their body water, and enter a dormant state called a tun. In this state, they have been revived after years, and they withstand temperatures near absolute zero, intense radiation, and even exposure to the vacuum of space, as shown in an experiment carried out on a satellite in 2007.

Scientists long assumed that tardigrades protect themselves with sugars such as trehalose, which many drought-tolerant organisms use to stabilize their cells. Yet tardigrades produce little trehalose. Research in the last decade has instead pointed to proteins unique to tardigrades, some of which seem to form a glass-like matrix inside cells as water leaves, holding delicate structures in place. Another protein, found in one species, shields DNA from radiation damage; when it was inserted into human cells in the laboratory, those cells suffered less harm from X-rays.

Such findings have led to excitement about practical uses, from preserving vaccines without refrigeration to protecting astronauts. Caution is needed, however. Tardigrades are not indestructible: in their active state they die from ordinary heat and dryness, and their endurance applies to a dormant phase rather than to normal life. More important, laboratory tolerance does not mean that tardigrades would thrive on other worlds. What they illustrate is that the limits of life are wider than once believed, and that the tools organisms use to cross those limits may prove useful to us.

What is the main idea of the passage?