Mouth is related to Eating. Meat can be used for Eating. Meat is the opposite of Plant. Peanut is a type of Plant. 
Letter is a type of Earn. Earn is related to Gain. Gain is related to Interest. 
Mouth <--RelatedTo--> Eating <--UsedFor-- Meat <--Antonym--> Plant <--IsA-- Peanut 
Letter --IsA--> Earn <--RelatedTo--> Gain <--RelatedTo--> Interest 