Here is what I found out:
A broody hen refuses to leave the nest and may cluck repeatedly when you go near her. She will only leave the nest once a day to eat, drink and defecate. Broody droppings usually come out in one large, very bad-smelling glob. Broody hens are most likely to happen in spring but can happen at any time throughout the year and is especially likely in the hen's second year. Broody hens do not lay eggs and they also take up a whole nest box. It can last over a month.
Many breeds of chicken have had the instinct to go broody bred out of them so they will produce more eggs.
To stop a broody early there are several techniques. Sometimes just putting her in a place where she can not see her old nest and keeping her there for 4 days will do the job. She should, of course, have feed and water. For the more stubborn hen, a wire-bottomed cage is necessary. The airflow up through the wire keeps her underside cool and after a few days she will usually give up.