Stop teasing the (nano)boy!
A rendering plant boils down flesh, gristle, bones etc. to make products such as fertiliser.
In Australian natural history museums, many of the exhibits are former zoo animals, and some of them have been there for decades. For example, Adelaide Museum has on exhibit (or did last time I was there) Adelaide zoo's first Asian elephant, who was named "Miss Siam" and who died in 1904.
Right near her is a Javan rhino which died in 1907 after 21 years at the zoo, during which time it was believed to be an Indian rhino. It wasn't until 1948 that a visiting American professor saw the body and told the authorities that what they had was, ahem, not an Indian but a much rarer Javan. Oops!