Smell is related to Gas. Gas can be used for Light. 
Gravity is related to Drop. Drop is related to Glass. Glass is related to Liquid. Spill is a type of Liquid. 
Smell <--RelatedTo--> Gas --UsedFor--> Light 
Gravity <--RelatedTo--> Drop <--RelatedTo--> Glass <--RelatedTo--> Liquid <--IsA-- Spill 