Definition is a type of Explanation. Key is a type of Explanation. Key is related to Access. 
Birth is related to Cake. Cake is related to White. White is related to Bone. 
Definition --IsA--> Explanation <--IsA-- Key <--RelatedTo--> Access 
Birth <--RelatedTo--> Cake <--RelatedTo--> White <--RelatedTo--> Bone 