Use the word "grain" when talking about the small seeds that are harvested from plants for food. Grain can come from wheat, corn, barley, rye, etcetera.
I prefer whole grain bread to processed white bread.
Most of the grain harvested in this area consists of corn.
Whole grain cereal is a good source of fiber for your diet.
Grain prices in the midwest are starting to come down due to an over supply of corn. (The laws of supply and demand are in effect.)
The excess grain that is harvested is stored in a grain silo.
Grain comes from seeds.
We also use "grain" when describing a very small amount of something:
There are a few grains of sand in my shoe.
Some grains of rice fell on the floor.
I always take what he has to say with a grain of salt. (This is an expression. It’s said when a person is suspicious of statements made by another.)
There’s not a single grain of truth in that statement.
The word "grain" is also useful when describing the patterns found in a tree that has been cut. The wood or lumber that is produced has a grain that might be desirable for making furniture or other objects.
Oak is valuable because it has an attractive grain.
Maple has a distinctive grain.
The grain in pine wood comes out when it is stained.
Cutting against the grain reveals the age of a tree.