Totally random. Some days, it seems like all of them are "difficult". But, those days do help your point average.
BTW, difficulty ratings are based on quote length, and have absolutely nothing to do with how hard they actually are.
Have you checked out some of the "help for newcomers"-type threads? There's loads of help in the forum if you take the time to look for it.
Slotted spoons don't hold much soup. - Stephen Sondheim