Terrarium

The Terrarium is an item used to make Cages for Critters caught with the Bug Net.

Cages cannot be opened, broken, or un-crafted, so once a critter is crafted into a cage with the Terrarium, that particular Critter can no longer be released.

Trivia

 * The plural of "terrarium" is "terraria", which is also the name of the game whose wiki you're currently viewing.