You find a chewed spool and see a thread trailing from your dog's mouth, or later from their rear. Do not pull it. A string, ribbon, or thread can become caught inside the digestive tract, and pulling may cause injury. Contact your veterinarian or an emergency clinic promptly with the amount, material, and timing if known. If your dog is choking, struggling to breathe, vomiting repeatedly, or in pain, seek emergency care immediately.
Keep the rest of the material and any packaging for the clinic. Do not offer food, medication, or a home remedy to "help it pass" without veterinary direction. A dog may seem normal soon after swallowing a foreign body and still develop a serious obstruction later. If the thread is attached to a needle, tell the clinic immediately and do not attempt to remove it yourself.
Prevent access afterward
Keep sewing supplies, gift ribbon, fishing line, and string toys secured out of reach. Supervise play with toys that can unravel and discard damaged ones. If your dog regularly eats non-food objects, ask your veterinarian to assess the pattern and seek help with management. Practice "leave it" with safe items only, and use doors or secure storage to prevent real string from becoming another test.
This is not a moment for a training experiment. The safe first step is veterinary advice while you leave the string alone.
Sources: Cornell University College of Veterinary Medicine: Gastrointestinal foreign body obstruction in dogs. PetMD: What to Do If Your Dog Swallowed Something They Shouldn’t Have.