Fixed Expressions

'Fixed Expressions' are sayings or phrases where several words that are often used together make one meaning. Some of them are more 'fixed' than others. This means you can't change the words without losing the meaning. Also some of them are more 'idiomatic' than others. An 'idiom' means that you can't understand the meaning from the words alone; you have to know the phrase to know the meaning.

As a matter of fact ('truth' doesn't work)
Idiom=60%, Fixed=100%

Honesty is the best policy ('rule' doesn't work)
Idiom=0%, Fixed=100%

Kick the bucket ('pale' doesn't work)('kicked', 'going to kick' does)
Idiom=100%, Fixed = 85%

Question: How do you find out what works?