Minecraft wiki entry for Time-saving_tips, associated text: Double-clicking an item attempts to pick up a full stack of it from the area you double-clicked in, then from other areas if it does not fill the stack. For example, if you double-click on a cobblestone block in your hot bar, it will pull as many as possible from your hotbar, then move on to your inventory, if the stack is not full yet. This is most useful for re-stacking buckets after you have emptied them of water or lava. Just double-click an empty bucket and you will grab 16 of them at once. You can then put the stack back in the hotbar, then double-click one in your inventory.