The Hindu Thread Trick by Lewis Ganson

The Hindu Thread, also known as The Gypsy Thread, is a classic of magic for a good reason. Audience reactions are very strong and it is extremely versatile by way of presentation. A thread is shown to the audience. One piece at a time, the thread is broken into several short pieces. The pieces are then rolled into a small ball. The performer grabs two of the pieces and pulls them apart, only to find the thread has been magically restored. Many of the top names in magic have performed this effect as a closer to their act.

It's that strong!