Shackle

A shackle is like a big, strong, metal "lock" that is used to hold things together or bound things to something else. It has a curved shape like a letter "D" and a little metal rod that can go through a hole and be locked into place. If you wanted to lock a bike to a tree so that no one can take it, you would use a shackle. The shackle goes through the bike's frame and wraps around the tree, then you put the rod through the hole and twist it, locking the shackle in place. This makes it really hard for anyone to steal the bike because they can't just lift it off the tree when they see it's locked up. Shackle is a very useful tool for keeping things safe and secure!