Put in the singular.
- They often go fishing and catch nothing.
- My friends like fruit, but do not like red meat.
- Babies are a great nuicance; they need attention all the time. If they do not disturb you by crying, they have to be fed, bathed or looked after. They do not even thank the people who take care of them. (In this sentence baby is a he)
- My children tell me that professors are people who think a lot but say little, and school teachers are people who say a lot, but think little.