The .io genre is one rule repeated: start small, consume what is smaller than you, become the thing that consumes. It got its name from a run of games on .io domains around 2015, and the shape stuck because it works on any screen and explains itself in four seconds.
Drain Rush 3D is the hole.io-style one — you are an opening in the floor, and you start by swallowing crumbs and finish by swallowing the plumbing. Turf Rush is the paper.io-style one: you leave a trail, close the loop, and the ground inside it is yours until somebody cuts the trail before you get home. Snake Sprint is the slither.io shape in 3D, where length is both your score and the reason you can no longer turn. Current Swim runs the same rule underwater with a current pushing you around.
None of these need an account and none of them hold you in a lobby. The thing a real .io game gets right is that the first thirty seconds are already the game, and that is the part we kept.
