Why are the insides of cats eyes sometimes skinny and really round?

You're likely describing the pupil of a cat's eye. The pupil is the black, circular opening in the center of the eye that allows light to enter. The shape of a cat's pupil can change drastically depending on the amount of light present:

* In bright light: The pupil will constrict and become skinny and round, like a vertical slit. This helps to reduce the amount of light entering the eye and prevent glare.

* In dim light: The pupil will dilate and become large and round, allowing more light to enter the eye for better vision.

This ability to change pupil shape is called pupillary light reflex and is a common trait in many animals, particularly those that are active during both day and night.

Here's why it's important for cats:

* Enhanced Night Vision: The large, round pupils in low light allow cats to see much better in the dark than humans.

* Protection from Glare: The thin, vertical pupils in bright light protect the cat's sensitive eyes from too much light.

So, the skinny, round shape you see is actually a sign of the cat's eye adapting to the surrounding light conditions.