Terrarium

The Terrarium is an item used to make Cages for Critters caught with the Bug Net. You can buy the Bug Net from the Merchant for 1 gold coin.

Used in
Some Critters, such as the Butterfly and the Firefly, cannot be put in Terrariums, but in jars and the Goldfish can only be put in a bottle of water.

Trivia

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