Unitialized constant

In Agile Web D. with Rails, I get this error when adding a
product to my cart:

uninitialized constant Cart

Application Trace:
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.2.5/lib/
active_support/dependencies.rb:200:in const_missing' ./public/../config/../app/controllers/store_controller.rb:21:infind_cart’
./public/…/config/…/app/controllers/store_controller.rb:9:in
`add_to_cart’

This error occured while loading the following files:
cart.rb

Any ideas what I did wrong? I can’t find a problem with “find_cart”
or “add_to_cart”. My cart.rb class has been created.

Thanks,
Doug