A horse chestnut is a large tree with spreading branches that produces shiny brown seeds inside spiky green capsules. These seeds look remarkably like edible chestnuts, smooth and mahogany-colored, but they're actually mildly poisonous and taste terrible.

Horse chestnut trees are common in parks and city streets across parts of Europe and North America, growing up to about 75 feet tall. In spring, they produce spectacular white or pink flower clusters that stand upright like candles. The leaves are distinctive too: each one fans out like an open hand with five to seven leaflets spreading from a central point.

Kids often collect the glossy brown seeds, called conkers in Britain, because they're so satisfying to hold and so beautiful when freshly fallen. The seeds have been used in a popular children's game for generations: you drill a hole through a conker, thread it on a string, and try to smash your opponent's conker with yours.

Despite the warning about eating them, horse chestnuts have been used in traditional medicine for centuries. Scientists have found compounds in them that may help with circulation problems, though people should not eat the seeds.