Well, yes, because 99 percent of people in general know that butter is made from milk, and that includes 100 percent of the people who might visit my house. And if I did somehow have a visitor so profoundly broken that they didn't know it, I would notice that.

Also approximately 100 percent of professional cooks know that butter is made from milk.

The butter in my fridge right now doesn't have labels on it. Because I take it out of the box it comes in and put an individual stick in that little covered shelf at the top of the door. My understanding is that that's what that shelf is for, so I suspect that this is a fairly common thing to do.

I certainly hope that anyone cooking food at a restaurant knows that butter is a milk product.