Suppose you have a glass of water on the table (the glass is half-filled, say). You accelerate the glass in a straight line across the table. The effect is that the water inside the glass rises up one side of the glass.
Why does this happen?
I suppose we can ask some other questions, like what is the shape of the water in the glass as a function of the acceleration, and what is the maximum acceleration of the glass such that none of the water spills out.