I recommend a moment of Zen before breakfast. Here's one for the holidays:

Consider the set S of all possible files of length n. A file compressor (such as Gzip, 7-Zip, and many others) is a programme that given any randomly chosen file from S, returns a new file whose size is on average greater than or equal to n.

Meditate on the statement above until you realise its truth. Then, grasshopper, you shall be enlightened.

Note: The "equal to" part is necessary because there is an entire family of compressors (the identity compressor being the most obvious one) that always return a file the same size as the one provided as input.