Commitment is a type of Message. Commitment is a synonym of Word. Word is related to Secret. Secret is a synonym of Mysterious. 
Ghost is related to Invisible. Invisible is related to Glass. Glass is the opposite of Metal. 
Message <--IsA-- Commitment <--Synonym--> Word <--RelatedTo--> Secret <--Synonym--> Mysterious 
Ghost <--RelatedTo--> Invisible <--RelatedTo--> Glass <--Antonym--> Metal 