Let’s see; I’ve seen them eat almost everything in the way of “people food” except for lettuce, breads, and most fruit.
“Cats have relatively few taste buds compared to humans. Domestic and wild cats share a gene mutation that keeps their sweet taste buds from binding to sugary molecules like carbohydrates, leaving them with no ability to taste sweetness.”
So why will they eat watermelon? My educated guess (I hold a BS in Biology - No, really!) is that they enjoy the cool refreshing aspect of watermelon.
Also from Wikipedia/cats:
“Cats are obligate carnivores: Their physiology has evolved to efficiently process meat, and they have difficulty digesting plant matter. In contrast to omnivores such as rats, which only require about 4% protein in their diet, about 20% of a cat’s diet must be protein. Cats are unusually dependent on a constant supply of the amino acid arginine, and a diet lacking arginine causes marked weight loss and can be rapidly fatal. Another unusual feature is that the cat also cannot produce the amino acid taurine, with taurine deficiency causing macular degeneration, wherein the cat’s retina slowly degenerates, causing irreversible blindness. Since cats tend to eat all of their prey, they obtain minerals by digesting animal bones, and a diet composed only of meat may cause calcium deficiency.
A cat’s gastrointestinal tract is also adapted to meat eating, being much shorter than that of omnivores and having low levels of several of the digestive enzymes that are needed to digest carbohydrates. These traits severely limit the cat’s ability to digest and use plant-derived nutrients, as well as certain fatty acids. Despite the cat’s meat-oriented physiology, several vegetarian or vegan cat foods have been marketed that are supplemented with chemically synthesized taurine and other nutrients, in attempts to produce a complete diet. However, some of these products still fail to provide all the nutrients that cats require, and diets containing no animal products pose the risk of causing severe nutritional deficiencies.
Cats also eat grass occasionally. Proposed explanations include that grass is a source of folic acid or dietary fiber.”
So just because cats can’t process vegetable matter doesn’t mean they won’t eat it, or that it will harm them. Just make sure that the house plants you keep are not poisonous to your precious fuzzies.