Word of the week: "To back up"

This week's word is the verb

To back somebody / something up

which means to support somebody or something; to say that what somebody says is true.

Examples:
I'll back you up if they don't believe you. 
The writer doesn't back up his opinions with examples.

This verb can also be used with the meaning of preparing a copy of a file or program which can be used if the main one fails. I'm sure you've all heard of a backup copy (using the word as an adjective). 

Example: It's a good idea to back up your files on a memory stick.