Final answer:
The phrase 'If you speak the truth, have a foot in the stirrup' is an idiom that suggests being prepared for the aftermath of speaking the truth. It is the only sentence among the given examples that contains an idiom.
Step-by-step explanation:
An idiom is a phrase or expression whose meaning cannot be understood from the ordinary meanings of the words in it. For example, "having a foot in the stirrup" is an idiom implying one is ready to take off or start an adventure. None of the provided text excerpts contain an obvious idiom except for the phrase from the fifth bullet point: 'If you speak the truth, have a foot in the stirrup.' This sentence suggests preparation for possible consequence after speaking the truth, and the expression is not meant to be taken literally, thus qualifying as an idiom.
Idioms add a colorful way to convey meanings and are commonly used in both spoken and written English. They're often cultural and can be confusing for people learning the language because the expressions don't relate directly to the words being used.