The catch? They need to be contained. Small metal items (bottle caps, nails, screws, and other pieces of metal 2 inches or less in length) should go in either a metal can crimped shut or you can ball them up in a piece of clean, used aluminum foil.
The key is to make sure that the can or foil is clean before you recycle it.
Metro has more about metal recycling at the curb and you can always call the Recycling Information Hotline (503-234-3000) if you have questions.