Strings, ribbon, thread, cassette tape, and yard are all examples of linear toys that many dogs will ingest. Some dogs or puppies will play with and eventually ingest these objects. They can get caught in their intestines causing a "linear foreign body" that can require surgery.
If your dog 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.