How a Bloom filter says "definitely not" using just a few bits

How can a computer check whether a word is on a huge list using only a few bits per entry? A Bloom filter flips a handful of switches for every word you add, then checks those same switches later.

It can say "definitely not" with total certainty, but "yes" only means probably. Add words, test strangers, and watch false alarms appear as the switches fill up.