A sac is a bag-like structure that holds or contains something, usually found in living things. Your body contains many sacs: tiny air sacs in your lungs hold oxygen when you breathe, and the sac around your heart cushions it as it beats. Plants have sacs too, like the pollen sacs that hold pollen until it's ready to spread.

Scientists use it to describe any enclosed, flexible container in nature. A spider's egg sac protects hundreds of baby spiders until they hatch. Some animals, like opossums, have a special sac where their babies stay safe and grow.

When you see sac in a science book, picture a small biological bag doing an important job: storing, protecting, or holding something your body or a plant needs. It usually refers to something natural, not human-made containers like backpacks or grocery bags.