If you visit the pet store looking for an appropriate type of pet litter or substrate to your line your pet rat's cage, you're going to find a large variety of different types of bedding. Options generally are broken down into those that made from wood and wood products, such as cedar, aspen or pine. Or those that are made from paper. Recycled paper products, paper that's put into pelleted forms or paper that appears to be just sort of like shredded or crumbled. There are also another varieties of what are called corn cob bedding or those that represent natural pieces like crushed walnut bedding. Any of those types. But when it comes down to making the right choice for your pet rat, I would strongly recommend considering one of two types. If you're going to use a wood shaving, aspen wood shavings are the best. They are the most absorbent, they don't tend to have any oils, they don't tend to have an odor and they tend to have the ability to keep the odor to the most minimum. So if you're going to choose a wood shaving or a wood product, I would use aspen.
If you're going to choose a paper product, I would choose a product like CareFRESH, and CareFRESH comes by the brand name and most of the large chain pet stores make their own version which can be a little less expensive. CareFRESH tends to be a recycled paper type product. It comes as either crumbles, it comes as formed pellets, or it comes as a crushed pellet that some people prefer to use. So going with these two types is the most popular, recycled paper type products do tend to be the most commonly used. They tend to be the most absorbent and most cost effective. They provide a nice substrate for your rat to climb in, to dig through. They're also highly absorbent and they're also very useful for the attractiveness of the cage as well. They tend to be lightweight and easy to change as well which I think is really important if you have a large cage or if you keep a number of pet rats.
Other options and this is contingent on the persons choice if they choose to have, would be towels. Some people prefer to use towels; they'll change them and launder them frequently. But they may want to use bedding. Fleece is an excellent material to use, if you want to use a material. You can get it different patterns, different colors, it's highly absorbent, it's easy to wash, and it tends to dry pretty quickly. But the majority of people that keep a pet rat are going to use the commercially available substrates to line the cage and generally the most common are going to be the recycled paper type products that are both absorbent, cost effective, and provide a nice, clean environment for the rat.