Ball And Ring
a.k.a. Ball And Chain
Old design, this one made by J. Storer, 1990.
(wood block, steel ring, and copper tubing, main loop is 6", block is 2.5", ring is 2")
The block fits through the big wire loop,
but not the ring,
and the ring can pass through the smaller wire loop.
Once solved it seems easy, but come back a year later and it can mystify you.
To get the ring off, pass it down through the metal loop at the base of the puzzle,
around the end where the string is attached, and back up to the position shown in the figure below.
Now continue the ring around the main loop and back through the loop at the base to come off.
If you make a mistake,
you can wrap the string twice around the ring rather than get it off!