Caravan question

When I use a caravan from one of my cities to another of my cities, I can move food or production. Is that actually taking food/production away from the home city, or is it basically just bonus food/production to a new city? I didn't see any drop in doing it, but wanted to make sure.

It adds production or food but doesn't take it from the originating city. The former requires a workshop and the latter a granary.