If you run regularly in Central Park, you've probably noticed black walnut trees if only because of their yellow fruit, which resembles racquetballs, strewn along the Central Park loop. The wood is beautiful and straight-grained. Black walnuts are considered delicious by humans and animals, though getting at the nutmeats is a challenge that can leave fingers deeply stained. Native to North America, the black walnut is an important commercial tree in the United States. The ground shells are used in many commercial applications, and during WWII were used to clean airplane pistons. The tree's roots emit a toxin that may be harmful to other plants; care must be taken in choosing what to plant near them.