A Mud pie is an equipable item that can be worn in the "weapon slot" of a player's equipment menu. It is because of this that the mud pie can be considered some sort of weapon, though it does no damage and is immediately lost when used (i.e. it's not stackable). Rather, it's thrown at other players to decrease their run energy by 50% (although it has no effect on NPCs). Players will get skulled if they throw it at a person they are not in combat with whilst in the Wilderness. It can be obtained by looting Baby implings or made using the Cooking skill. You will need to have level 29 cooking to bake it, and will receive 128 cooking experience from doing so.
Mud pies are needed in some treasure trail clues
Making a mud pie
Mix flour and water to make some pastry dough.
Place the dough in an empty pie dish to make a pie shell.
Add compost to your pie shell to make Part mud pie 1.
Add water to the pie using a bucket to make Part mud pie 2.
Add clay to the pie to make a Raw mud pie.
Cook on a range.