Minecraft wiki entry for Mob_farm, associated text: In Bedrock Edition placing water on top of scaffolding provides an efficient means of active transport. Land mobs spawn with their feet inside of the scaffolding and their heads inside of the flowing water. The mobs are then immediately transported to a drop chute or other killing area. Spiders and baby mobs present a challenge for water-on-scaffolding designs because they spawn entirely inside of the scaffolding and cannot be moved by the water until they randomly climb/jump. Several methods have been invented to deal with this: buttons placed every-other block in every-other row prevents spiders from spawning at all; villagers can be used to lure baby zombies (see below); snow golems can be used to knock mobs upward and into the flowing water; or cascading water down a funnel-shaped spawn floor can be used to pull babies and spiders into the water flow at every block.