Sorry is related to Heat. Heat is related to Son. 
Volunteer is similar to Voluntary. Voluntary is related to Willing. Willing is similar to Ready. Ready is related to Complete. 
Sorry <--RelatedTo--> Heat <--RelatedTo--> Son 
Volunteer <--SimilarTo--> Voluntary <--RelatedTo--> Willing <--SimilarTo--> Ready <--RelatedTo--> Complete 