This lid uses water to form an air trap that will let carbon dioxide and other gasses vent off of fermenting foods, while keeping out the oxygen that would spoil them.
Some ideas for fermented foods this could be used for include sauerkraut, kimchi, ginger beer, or pickled vegetables (garlic, carrots, peppers, etc). Fermented foods are healthy, easy to prepare, and taste delicious.
This was inspired by this: pickl-it.com/ and motivated by this: thingiverse.com/thing:4663 and my completion of a general purpose canning jar lid (here: thingiverse.com/thing:19105).
The air trap is one piece, with no moving or meshing parts.
This thing is a part of a collection of things featured in this post: protoparadigm.com/blog/2012/08/what-to-do-with-a-3d-printer-make-canning-jar-accessories/