What do I say about why I left my last job? - 15 weeks in

They always ask why I left, and I freeze. I have read a lot of threads here and could not find my exact situation. I also do not want to seem like a risk. What do you say?

Say it plainly and move on to what you can do. Interviewers usually move on if you do.

Keep everything in writing and save the confirmation numbers. It saved me later.