Minecraft wiki entry for Combat, associated text: Cacti can be used to make effective traps against mobs. When in contact with the block, any mob (including the player) will lose half a heart every half seconds, however, mobs may avoid them, but if the player wants to make a trap, the player can place three cacti plants, two blocks high, in a V format. The player might want to lure them nearby (around 10 blocks far from the trap) and hide in the opposite side, where they will try pursue players but will not be able to reach.