a structure is a kind of feature & a part of a living thing is a natural structure ->  a part of something is a feature of something  a part of something is a feature of something & roots are a part of a plant ->  roots are a feature of plants  grass is a kind of green plant & a tree is a kind of plant ->  a tree and grass are both kinds of plants  roots are a feature of plants & a tree and grass are both kinds of plants ->  roots are a feature of both trees and grass  roots are a feature of both trees and grass & if two things have the same features then those two things are alike -> a tree and grass are alike because they both have roots