balls make great nests tied upin cage but you need to be careful if you use them as intended. when a rat is placed in one you are effectively blinding them. they see mostly with their sense of touch and smell both of which are inhibited in a ball. they could easily end up rolling off a ledge or down stairs and could be hurt. so caution is advised. i'm not sure the specifics of why DFW's rat broke his leg but she was lucky that was all the happened, i've heard of worse.
also, those balls are not the most secure things in the world. i've had a hamster get out of his before. if it wasn't for my cat's motherly tendancy and odd habit of taking on rodent children i may have lost the hamster. instead, sesshoumaru dropped fearless in my lap for safe keeping.