Difficult is related to Tough. Tough is related to Iron. Iron is related to Clothing. 
Dirty is related to Pig. you are likely to find Pig in/at/on Mud. Mud is a type of Dirt. 
Difficult <--RelatedTo--> Tough <--RelatedTo--> Iron <--RelatedTo--> Clothing 
Dirty <--RelatedTo--> Pig --AtLocation--> Mud --IsA--> Dirt 