Strings, ribbon, thread, cassette tape, and yard are all examples of linear toys that many cats love. Some cats will chase and play with these objects for hours.
However, these objects can be deadly if ingested. Some cats will start to play and end up eating these objects. They get caught in their intestines causing a "linear foreign body" that can require surgery.
If your cat likes to play with these types of objects – be very careful. Ideally, only allow him to play with them when supervised. If there is any sign of him wanting to chew on or ingest the object – take it away. Be very careful not to allow for these items to "lay" around the house.
For more information please read Safe Toys for Cats
or Gastrointestinal Foreign Bodies in Cats