Is my pet snake too fat? Or too thin?

Because we humans eat every day, it's difficult for us to get our head round the fact that reptiles don't need to. They need less food than we do because they don't have to heat up their bodies - the sun does that for them.
So many pet snakes end up too fat. Sometimes this is because their owners continue feeding them the same amount of food throughout their life. When they are young and growing, they need more than when they get to middle age - just like humans!! So if you feed your mature snake as often as you did when it was young, the result will be a portly animal.
With corn snakes, the fat is likely to accumulate just above the tail - look at the photo on the right and you will see the tail is growing out of a kind of bulge. That's usually a fat snake. If however it is an animal you have just bought, or if that lump occurs suddenly, then you need to take your snake to a vet to make sure it isn't some kind of illness.

The other mistake is not to feed your snake enough. That often occurs because if a reptile is kept at the wrong temperature without enough heat, their whole metabolism slows down. They lose their appetite and just go off their food. Slowly, their colours fade, their spine or ribs start showing and they become weak with starvation. Many of them will be dying before they get to a vet.
So how often should you feed your snake? How much? The detailed answers to this question depend on the snake's species. Get a good online care sheet from Evolution Reptiles here or from the RSPCA. Don't just rely on friends or forums  because the quality of advice will vary.
Finally, how do you decide if your snake is fat or thin or just right. Take a look at the very rough sketch below. It shows a cross section of a snake. The triangular shape on the left is a snake that is too thin - you may be able to see an outline of the spine at the top or even its ribs. The snake in the middle is the right size, a sort of dome-like shape with a flat bottom which is where its muscles pull it along the ground. The snake on the right is round all over - and too fat.