I know that many parents find a teen's imploring "But..." to be the thing which gets on their very. last. nerve. Personally, I encourage the use of the word. It introduces a contrasting statement and shows thinking. I need kids to stick out those buts and share their ideas.
As for me? I try to avoid it like the plague. Why the double standard? Because as soon as a kid hears that word leave your lips, they tune out. No matter what good you've said beforehand, a teacher's big but will undo all of that every time.
"What you did with your hypothesis was very good, but your conclusion needs work."
This is not to say that kids don't need to hear constructive criticism and understand where they can improve. It is our responsibility as teachers to keep students moving forward with their learning. We just need to be sneaky with it by asking kids questions instead.
"This hypothesis is good because you state a reason to support the prediction. Another scientist might wonder if you still think the same reason would apply to your conclusion. What would you tell him or her?"
I've been trying to do this more this year and keep my big but out of student work. So far, I'm pleased with student response to the results. Many are using the comments to make positive changes in their work and I think it has helped nurture the classroom environment. We're not looking for what's wrong, but we're taking what's right making the rest of it better.