Currently, if you fill a teapot with the wrong liquid, there's no way to remove it. I think that either by dumping it in the world or by crafting it in an empty grid, we should be able to remove the liquid from the teapot.

Why would you ever dump out tea? Such a waste.

For the water/milk case, I probably could just implement the water cup sprite Elucent made for a water bottle like mechanic. So let you just pour the teapot into cups to empty and get water/milk cups, then drink the cup. The cup is needed as drinking from a tea pot directly is gross.