multiple categories for a product

I’m currently using acts_as_tree to handle my categories hierarchy.

Animals
Birds
Dogs
Themes
Farm
City

My product model currently just has a “product_id” column, with an acts_as_tree object and belongs_to :category

If I wanted to define a product that belonged to more than one category
(ie “Rooster” – could belong to both Bird and Farm categories simultaneously), what is the best way to do this?