Trampoline

The trampoline was a human invention on Earth. It was made for people to bounce on, for exercise or amusement.

Whilst on Platform One in the year 5000000000 Rose Tyler called Cassandra O'Brien "A bitchy trampoline" after declaring that Cassandra would have done better to have died in her natural time than live as she did; a piece of stretched skin. (DW: The End of the World)

23 years later in 5000000023 Cassandra used the derogatory name herself, as she invaded Rose's body, and thereby finally ceasing to be flat. (DW: New Earth)