Milk is the fastest liquid on earth

Milk is the fastest liquid on earth – its pasteurized before you even see it

This is a pun - the critical part of the humor is the word pasteurized. To pasteurize something is to heat it up in such a way that it kills the bacteria in it. Most milk that is sold in the US is pasteurized. The word pasteurized also sounds like "passed your eyes", and if milk moved so fast that you were unable to see it, it would be moving pretty fast.