I just finished a question in which a u-shaped tube (manometer) is filled with water to a certain height. Another substance, in this instance, oil, is added to one side of the manometer. I was asked to find the height of the new substance. I used the formula P(water) = P(oil), or (ϱwater)gh = (ϱoil)gh. So, I know how to solve it, but I don't really understand it. Why is the pressure of both substances the same? Thanks for your replies.