On its / it's

This is one of those common errors that drives me nuts -- but it’s easy to remember that only the verb in this pair has its own apostrophe.

Its is a possessive pronoun that indicates ownership:
What is the cow chewing? The cow is chewing its cud.

It’s is a contraction built from it + is, and it indicates that a thing is doing something (or even simply that the thing exists):
Where is the cow going? It’s going back to the barn.

Put them together:
Now that the cow has chewed its cud for an hour, it’s time to go back to the barn.
(This means that after the cow has chewed its own cud, it is time to head for the barn.)

To remember the difference, try these clues:

• Its is like mine or yours – they are all pronouns that tell who owns something, and none contains an apostrophe (’).

• It’s is a verb built from it + is, and if you can break down your word into it is and still make sense, it’s with the apostrophe (’) is the one you want.