  Liquids flow, making them remarkably distinct from solids and close to gases.
At the same time, interactions in liquids are strong as in solids. The
combination of these two properties is believed to be the ultimate obstacle to
constructing a general theory of liquids. Here, we adopt a new approach to
liquids: instead of focusing on the problem of strong interactions, we zero in
on the relative contributions of vibrational and diffusional motion in liquids.
We subsequently show that from the point of view of thermodynamics, liquid
energy and specific heat are given, to a very good approximation, by their
vibrational contributions as in solids, for relaxation times spanning 15 orders
of magnitude. We therefore find that liquids show an interesting {\it duality}
not hitherto known: they are close to solids from the thermodynamical point of
view and to gases from the point of view of flow. We discuss the experimental
implications of this approach.
