Carrying around your own coffee mug on Friday could prove lucrative, while also benefiting Mother Earth.

Coffee giants Starbucks and Caribou are among the retailers offering freebies and other specials to customers Friday in observance of Earth Day. Both are giving away free coffee to people who bring in their own mugs, and Starbucks is also offering free tea.

Caribou normally gives mug-presenting customers 50 cents off, while Starbucks ordinarily gives 10 cents off.