A sac is a part of an animal or plant that is like a bag and holds something.
A sac is usually thought of as a small, soft, and delicate thing.
Examples:
The bee has a sac for honey.
The sac of the plant holds the seeds.
The sac is full of water.
The sac is like a little bag for the plant.
The sac is as big as a house!
The sac is very important for the animal to live.