How do I know what songs I buy off of iTunes are ringtone-able?

I bought Master of Puppets by Metallica and planned on using the intro to that as my ringtone but when I clicked Create Ringtone... it said that it was not available (or something along those lines). I then tried doing it to I Remember You by Skid Row which I bought about 2 weeks ago and it worked. I'm a little confused as to what works and doesn't work.

It doesn't seem to be how long ago it was purchased because I clicked MoP literally seconds after downloading it and it didn't work but I Remember You worked after around 2 weeks. I also tried Panama by Van Halen and it wouldn't let me do that one either and that was purchased the same day as I Remember You.

Any help would be greatly appreciated. Thanks