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. 
Flood is a type of Fill. Flood is a type of Light. 
Bus <--PartOf-- Roof <--AtLocation-- Water <--DistinctFrom--> Grass <--RelatedTo--> Sheep 
Fill <--IsA-- Flood --IsA--> Light 