Parent's Bitter Coffee

At visiting my parent's this holiday weekend I tired to gently tell my parents that their coffee brew is very bitter. The response I get back is something like, " I like it strong."

I wasn't too sure how to respond, but then they told me my coffee is to watery. 🤔 I told them it's not that is watery, but it's a light roast and not bitter tasting.

So my question is how do you convince someone that bitter coffee is not good coffee? I might bring my scale next time to help measure and perfect the coffee brew there. Maybe even see about cleaning their been grinder, which I think has never been cleaned.

Oh well.

Update: Thanks for all the tips and thoughts. I agree with basically everything posted here and sorry no butter (I fixed the title)