Chemistry - Entropy - "Wiggle"?

You can't measure entropy directly, any more than you can measure interatomic distances. You measure other quantities -- for instance often you can measure energy gain/loss and temperature, and then you integrate $dS=dE/T$.

How to explain it? One of the best expositions I know is The Second Law by Henry A. Bent. It is full of insightful examples, lays the ground carefully and avoids woolly talk, unlike many other thermodynamics books.

Some elementary but valid comments about the link between information-theoretical entropy and thermodynamical entropy can be found here:

Edit: Greg reminds us that energy and enthalpy are not directly measurable either. That's why I was careful to write energy gain/loss, which are somewhat more accessible to measurement; but of course even when doing a calorimetric experiment you're not measuring heat per se but other quantities: how much gas you burned or how intense the current was and how long it was left on. And even some of these quantities again are indirectly measured.

I agree 100% that "entropy is not more abstract than energy" -- they are all abstractions. Ultimately, however, I do think the measurement of thermodynamic entropy is one step more indirect than that of energy, if only because it involves the notion of temperature, which (if you think about it carefully) is subtle indeed.