Why is it impossible to “unmake” a cup of coffee? This is because of a concept in thermal physics, called entropy. So what is entropy actually? It is a strange idea that pops up in seemingly unrelated conversations about information, disorder, thermodynamics, black holes and time

Entropy is a way of describing a larger system by looking at the interplay of it's individual smaller components. That sounds rather vague, and indeed it is. It's vagueness is partly why it appears in so many diverse fields.

To understand entropy, one must understand the concept of irreversibility. We can understand this by looking at the second law of thermodynamics, which states that heat will always flow from hot to cold.

Combining this law with our notion of entropy allows us to obtain a truly fascinating result. It is known as the arrow of time. It is the only law of physics that allows us to determine which way time flows. This is why it's impossible to "unmake" a cup of coffee.

It is also why we are able to tell when a movie is played in reverse (except when Christopher Nolan is involved)