Minecraft wiki entry for Hopper, associated text: Item streams, on the other hand, are made by running water over the hoppers and dropping items into the flow with a dropper. Whenever the stream grows too weak, ice and blocks that stop water flow but not moving entities (such as signs, open fence gates, top slabs/trapdoors, buttons, etc.) are used to carry items across the breaks. This is a cheaper method, provided Silk Touch has been obtained. However, while items are flowing through the system, it may generate more lag than using hoppers. Additionally, if too many items move past a filter at one time there is a chance for items to skip over and not be sorted. In Bedrock Edition edition, any item in collection range of a filter hopper that does not match the filter item in the hopper can block the hopper from collecting a matching item.[2] Hoppers also have a collection cooldown that limits them to collecting only one group of items every 8 game ticks.