Physical or electronic objects are encoded with identifiers, which serve to trigger object-appropriate responses from computer systems that encounter such objects. The encoding may be steganographic (e.g., by digital watermarks), so the presence of such identifiers is not evident to persons encountering the objects. An exemplary application is a computer system that looks at a printed magazine advertisement (20) and initiates a link to a corresponding internet page. In one such implementation, the computer system senses an identifer encoded in the advertisement, forwards the identifier to a remote database, receives from the database (17) a corresponding internet address (18a, 18b, 18c), and directs a browser to that address (18a, 18b, 18c). The same arrangement can be used for on-line ordering from printed merchandise catalogs. Another application is a computer system that looks at a printed spreadsheet (20), and retrieves from disk storage an electonic version of the same document for editing.