Each button, field, card, background, and stack has a script, which you can edit. The script contains text that describes what messages the object understands and what actions should happen in response to the message.
-- part contents for background part 22
----- text -----
453,238
-- part contents for background part 3
----- text -----
HyperCard carries out the script in response to messages such as "mouseUp". See also: message.*