Very popular and very easy!
An ordinary length of rope is fed through the hole in a plastic block.
The rope can be seen through a cutout window in the front of the block.
When the two halves of the block are pushed apart, the rope can clearly be seen to be split into two separate pieces.
Yet when the block is put back together, the rope is restored and can be pulled out of the block in one piece!
So simple, this is an excellent trick for the new magician.
Shows up well for a small group or classroom too!
This all-time favorite classic pocket trick is now manufactured in heavy duty plastic for durability.