Two players take turns placing stones on empty spaces of an initially empty hexagonal grid of hexagons, one stone per turn. There must never be a stone adjacent to six other stones on the board. You win if, after your placement, there is an empty space whence the edge of the board cannot be reached through a series of steps onto adjacent empty spaces. The empty area including that space is then said to be looped.
If you play several rounds, note after each round the size of the looped area and add it to the winner's running score. Whoever reaches a predetermined score first wins the match. The loser of one round plays first in the next.
Like all impartial games, Luup is exquisitely boring. To make it partisan (if you must), fill a looped area with markers of the looper's color and keep playing until there are no more legal moves available. If you loop an area, you must make another move on the same turn. Whoever has more markers in the final position wins.
As an alternative to the standard placement restriction, one may forbid non-perimeter placements that complete minimal triangles of stones around a vertex of the grid. This is a bit less elegant but possibly more fun and accessible. Loops are guaranteed in both cases.
—description from the designer