They know it's that old because they killed it to find out. Meanwhile, a 116-year old orange is put on display in Britain. It
belonged to Joseph Roberts who was injured in an explosion at a Stoke-on-Trent colliery in 1891.
But his lunchbox survived.
This article available online at:
http://www.theatlantic.com/daily-dish/archive/2007/10/a-405-year-old-clam/224075/
