If I tell you something you already know (e.g., "The sun will rise tomorrow"), I have transmitted very little information. If I tell you something shocking (e.g., "The sun did not rise today"), I have transmitted a massive amount of information.

Why the logarithm? Because information is additive. If you flip two coins, the total surprise is the sum of the individual surprises. The logarithm turns multiplication of probabilities into addition of information. The most famous equation in information theory is Entropy ( H ):

When your data corrupts, you are witnessing a violation of the Hamming distance. When your compression algorithm bloats instead of shrinks, you are witnessing low entropy.

In Shannon’s world,