Tighten is a type of Change. Strain is a type of Tighten. Strain is a type of Disease. Disease is related to Medicine. 
Find is a type of Reason. Reason is related to Birthday. Birthday is related to Celebration. 
Change <--IsA-- Tighten <--IsA-- Strain --IsA--> Disease <--RelatedTo--> Medicine 
Find --IsA--> Reason <--RelatedTo--> Birthday <--RelatedTo--> Celebration 