Structures and their jobs
Each part of a plant or animal is shaped for a particular job, so its shape tells you its use.
A worked example
Webbed feet spread wide to push more water, so the duck swims. A long thin beak reaches into narrow places a short thick beak cannot.
The mistake people actually make
Roots take in water and hold the plant steady, but they do not make food. Food is made in the leaves, using sunlight.