Toxicodendron radicans
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If you look up into a tree and see 3-parted leaves, inspect the trunk for a really "hairy"/ rooty vine growing up it, and DON'T TOUCH. Eastern poison-ivy is severely irritating to many people's skin.
The vine can also grow along the ground, sending up leggy shoots.
Eastern poison-ivy's leaves could be confused with box-elder's 3-parted (sometimes 5-parted) leaves by their similar shape. Look at the arrangement of the leaves to be sure: poison-ivy's 3-parted leaves grow staggered along its climbing stems. (By contrast, box-elder's twigs grow in pairs along its branches.) Poison-ivy has green berries that turn white by autumn. (Box-elder has winged seeds like a maple tree.)