What do platypuses' tails do?

The tail of a platypus helps the animal to swim and acts as a rudder when it dives.
A platypus also stores most of its body fat in its tail to help it survive when food is limited, and to prepare for when the female must incubate her eggs and care for the young when they hatch.
During the breeding season, the female digs herself a chamber within her deep burrow, and this chamber is lined with leaves and other nesting material. The female with carry this nesting material rolled underneath her tail as she climbs up and down steep riverbanks.