What attracts maggots?

Maggots are insect larvae, usually hatched from the eggs of houseflies (or other flies) that have been laid in edible material (e.g. carrion or rotting meat).

Because they seem to come from "nowhere", originally many people thought they appeared spontaneously from rotting meat. Especially since maggots can hatch out in closed containers, the invisibly-small eggs having been laid before the contents were put in the container.

(see related link)