I've only needed this trick twice in my Rails life, but both times I had to google for it so I figured I'd post a note to remind myself for next time.
Sometimes you want to store an array as part of your model, and not make it an index into another table. ActiveRecord allows you to serialize a property to do just that:
class MyStuff < ActiveRecord::Base
In your DB schema, you'd want to make "things" a text value. Then you can do stuff like this and not worry about anything:
stuff = MyStuff.new
stuff.things << "laptop"
stuff.things << "ipod"
Done! Arguably this is a bit of a hack, as the next natural question to ask is "Well isn't there a value in cross referencing all the items that could be in "things"? Could be. Depends on the application. It's nice to know that if you don't want to go down the path of setting up that extension to your model, you've got an easier option.