Evaporative Emission (EVAP) Canisters
WARNING: This page is about a different car, the 2004 Mercury Monterey and 2004 Ford Freestar. However, it is still accessible from the selected car via links, so may be relevant.
Fuel vapors from the fuel tank are stored in the EVAP canisters. When the engine is running, the vapors are purged from the EVAP canisters for combustion.