"Aoccdrnig to a rscheearch at an Elingsh uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht frist and lsat ltteer is at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae we do not raed ervey lteter by it slef but the wrod as a wlohe. ceehiro."
I'm sure it has more rules than that. I read that before, tried my own with other texts and realized it doesn't always works as well as that text.
If the rearrangement of the letters in the middle fits some conditions, the word can become unreadable. That's because several words have the same first and last letter, and similar in-between letters.