Bubble

A bubble is background decoration that liquid treats as a wall. You walk straight through one, enemies and critters do too, but water, lava, honey and shimmer all stop dead against it. Every clever build people make with these comes out of that single reversal. If you want the opposite behavior, where liquid passes and bodies do not, that is what a grate is for.

They will attach to each other with nothing behind them, which is the point. Put a cluster up against a wall or a floor, then mine away whatever was holding it, and you are left with a shape floating in open air. Joined bubbles merge visually into one larger blob rather than reading as separate tiles. They cannot be hammered, so there is no sloped version and no way to make a liquidfall out of them, and block swap refuses to work on them, meaning you have to mine one out before anything else can go in that spot. Actuators are inconsistent too, ignoring plates, switches and levers while responding to timers and sensors.

Bubbles count as valid frame tiles for housing while still letting you walk through them, so a room built from them has no entrance to open and nothing for an enemy to break down. That is worth the material for a Nurse room in an arena, where the whole value of the visit is how fast you can get in and out. They do not satisfy the interior surface a house needs, so the room still has to be floored with something real. Town residents will pass through empty bubbles and ones holding water or honey, but they refuse to step into a lava filled one, while critters and enemies walk through those quite happily.

Each bubble holds its contents privately. Two touching bubbles do not share liquid and do not let their liquids react, so a small square of them can store water, lava, honey and shimmer side by side with nothing hissing or turning into obsidian. Fill one by clicking it with a full bucket, take the contents back with an empty one, and mine the tile if you want it dumped on the floor. A filled bubble behaves exactly like the liquid inside it: you can swim in a water one, drown in it, or walk across the top with water walking gear. The one thing that breaks the isolation is a slower liquid flowing in from outside, which consumes the whole tile at once.

Items are not entities. They pass into a bubble the same way you do, which means a single suspended tile of lava still destroys any coin or drop that falls into it, however thin the layer looks. A lava filter hung over a farm eats the loot. Lava also destroys the bubbles themselves, so the wall of your trap is part of the trap. And a lava ceiling built to catch flying enemies will kill every butterfly and firefly that drifts under it as well.

Honey pockets scattered around an arena hand out a regeneration buff on the way past with no plumbing at all. Water pockets do something similar for the Cute Fishron mount, which gets a speed burst on contact with water, so a line of them across the map becomes a boost track. On built adventure maps they work as liquid gates, holding back a lava flow that visibly wants to pour through an opening you can still run into.