Conditional Probability

What's the chance you have a cold? Small — say 2% on a random day. Now add a symptom: you're sneezing and feverish. Suddenly the chance jumps to 60%. Nothing about the cold changed — what changed is what you KNOW. Conditioning on evidence shrinks the universe from 'all possible days' to 'days with sneezing and fever' and re-measures the cold inside that smaller world. This is how doctors diagnose, how spam filters decide, and how every detective narrows the suspect list.