I have an insect/mosquito zapper. I want to attract the mosquitoes to this zapper to kill them. The best way to do this is by producing carbon dioxide next to it. I'm thinking that if I put a jar or bucket of some liquid (or other substance) that gives off a non-dangerous amount of carbon dioxide it will attract the mosquitoes to that area and the zapper will do the rest.

So what is the best way to produce carbon dioxide for this purpose?