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Droplets are scripts that can have files (or folders full of files) dropped onto them and have the script work on some, or all, or the items that were dropped onto it. A droplet's icon will show a downward arrow suggesting files should be dropped onto it.
To create a droplet you must do two things: use the "on open" command around the body of your script and save the script as an application. Here's an example:
on open theFiles
tell application "Finder"
repeat with aFile in theFiles
set comment of aFile to "Groovy!"
Files dropped onto this script will immediately change its comment to "Groovy!"! Using the same approach, all kinds of things can be done...changing creator codes, making backups, etc.