To untie a knot.
Untie a horse from a tree.
It was tempting to untie the twine from her waist and wander a little further, but the forest was too confusing.
Plus, there's no worry that your little one will untie his shoes while you're not looking.
Let's untie that knot in the elephant's trunk together!
You can also untie them when you want to lay out and tan, not worrying about the presence of lines.