Charles Sanders Pierce distinguished three basic types of signs: icons which resemble their object, indexes which have a direct connection to their object like smoke indicating fire, and symbols which connect to their object through convention like words. Semiotics is the study of signs and symbols, and it examines their use in advertising where some cultures employ more symbols than others. Important signs and symbols in advertising include packaging, colors, letters, and numbers which form association networks in memory and can have strong cultural meanings.