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Tardigrades, sometimes called water bears, are microscopic animals famous for surviving conditions that would kill almost anything else. When their surroundings dry out, they pull in their legs and curl into a compact ball known as a tun. In this form, the water in their bodies drops from roughly 85 percent to under 3 percent, and a sugar called trehalose spreads through their cells, shielding delicate structures in place of the missing water. A tun can remain dormant for years, then revive within hours once moisture returns. This trick, not any toughness while active, explains their reputation for near-indestructibility.