You have to but a wheel made specially for rats, here they are expensive. It has to be made for rats because their tails or feet get caught in them and they could break. A rat wheel looks just like a normal wheel, but there is no spaces in between the bars. Here is a pic of one.

[img]http://rds.yahoo.com/S=96062883/K=rat+wheel/v=2/SID=e/l=IVI/SIG=11t1gkhlf/EXP=1118776716/*-http%3A//www.transoniq.com/images/rat25.jpg[/img]