Today’s saying is:
To lose one’s train of thought
It means:
To forget what you were in the middle saying or thinking.
Examples of how to use it:
I was in the middle of training my new employee on how to use our company’s software when I totally lost my train of thought; so I took a break and got some coffee before continuing.
He lost his train of thought in the middle of his speech, but nobody really noticed, since he picked right back up again.
I keep losing my train of thought; maybe I should take a nap.
What was I just saying? I lost my train of thought. Oh yeah, now I remember. I was telling you about last night’s dinner.