Roof is part of Bus. you are likely to find Water in/at/on Roof. Water is a different concept from Grass. Grass is related to Sheep. 
Congressional is related to Page. Page is related to Dirty. 
Bus <--PartOf-- Roof <--AtLocation-- Water <--DistinctFrom--> Grass <--RelatedTo--> Sheep 
Congressional <--RelatedTo--> Page <--RelatedTo--> Dirty 