I agree with Becky. Your ball can feed on mice its entire life and be perfectly happy. However if you are insistant that it converts to rats tehn I have a couple of tricks I tried on my burms switching them to rabbits(no one wants to keep a burm on rats...lol). First try a rat hopper small....a live one. Drop it in the cage, shut the lights off and leave it alone for about 30 minutes or so. One method that has always worked for me is what I call chasing. I feed the snake a small prey item of their liking, in this case a mouse. Make sure it is smaller then normal. Then snake will take the mouse and start swollowing it. Heres the tricky part, Take a small prekilled or ft rat and just as the ball is gulping down the last bit of the mouse gently insert the head of the rat into its mouth. If you work it in right the snake should continue on eating and continue on to the rat. You usually just have to get the head into its mouth and it will work. Usually have to do this two or three times but the have always gotten used to the new prey item this way. Hope this helps, Todd