Bloom filters can very quickly answer variations on the Yes/No question “is this item in the set?”, like “have I seen this item before?”. There are two important caveats though. Very rarely, it will say Yes when the answer is actually No (although it will never say No, when the answer is actually Yes). You also can’t remove an item from a Bloom filter. Like elephants and unrelenting Polish designers, Bloom filters never forget.