The female tabby (number four) eating cat grass. Cats do eat grass, and it seems they like it, but in fact there are only theories as to why they do. Some claim it is to get extra niacin, a B vitamin abundantly available in most fresh young grain grasses. Perhaps cats eat it to make themselves vomit. Some people believe cats eat it to help pass fur balls along while others say they just need the fiber for other nutritional purposes.