here's the way I heard it:

The zoo at Mersey is famous for its collection of koala bears. As you know, koalas eat only eucalyptus leaves. But one day the painters working on the cages spilled paint on the leaves, and the koalas refused to eat them. The painter finally found a solvent which removed the paint without killing the leaves, and reported proudly, "The koala tree of Mersey is not stained."