Chalk is a type of soft white rock. You can use small pieces of it for writing or drawing with.

Chalk is small sticks of chalk, or a substance similar to chalk, used for writing or drawing with.

If you chalk something, you draw or write it using a piece of chalk.

If you say that two people or things are like chalk and cheese, you are emphasizing that they are completely different from each other.

You can use by a long chalk to add emphasis to something you are saying.