That idea sticks on your head because it follows the book's recipe - it is a :
- unexpected (alien?),
- concrete (my brain? ARGH!),
- credible (ok, you probably WON'T meet any actual brain-borrowing aliens, but you understand the mechanical concept),
- emotional (ICK and EUW!)
Read the book, find out how you can make brain worms too! It's fun and useful!