Construction Foam is the ultimate material against mobs and other players.
After it's placed, it will eventually turn into a Wall.
The process of wall-solidifying can be sped up by placing a light source near the foam or performed instantly by right-clicking with a block of sand.
In the meantime, the foam can be walked through by anything, including mobs (although the risk of suffocation or getting stuck in a new wall is high).
Note: the recipe is shapeless.
The Foam when solidifies it turns into Wall which is Anti-Explosive. No more Creeper accidents! HAYO!