Aberdeen Thread by James Dickson

You reveal a length of thread and give to the spectator to examine. You break it into multiple pieces. The pieces are shown separate, one-by-one, then rolled into a ball. Slowly, you pull the ends of the string to reveal that it has been fully, impossibly restored. The spectator can examine everything - and keep the restored thread as a souvenir.

The Gypsy Thread dates back to 1584. The plot was mentioned in The Discoverie of Witchcraft, one of the first books on magic ever published. In the past, the effect was somewhat cumbersome and required a significant amount of setup. ABERDEEN annihilates all of that.