Rugae are the ridges or folds in the stomach or the mouth that can stretch and shrink.
Rugae help the stomach and mouth do their jobs better.
Examples:
The rugae in the stomach help it stretch when we eat a lot.
When you stick out your tongue, you can see the rugae in your mouth.
The rugae in the mouth help us eat.
The rugae in the stomach are like the wrinkles on my grandma's face.
The rugae in the mouth are like the hills and valleys on a map.
The rugae in the stomach are like the springs in a bed.