Notched wooden sticks interlock into one solid block, no glue involved
A burr puzzle is built from several notched wooden rods, each cut with its own sequence of slots, that slide together into a single interlocking solid, often a cube with short sticks jutting from each face. How do a handful of straight, rigid pieces lock into one solid shape with nothing holding them but their own notches?
Reveal the answer
The classic form is the six-piece burr, but far more complex versions exist whose solution requires a precise sequence of moves, sometimes dozens long, where a piece must be partly withdrawn, rotated, and reinserted before any other piece can move at all. The puzzle family's exact origin is undocumented, but burr puzzles have circulated in Europe since at least the 18th century.