When you are writing down notes and don't want others to see some mistake you have written down, instead of just scribbling over the letters which only partially hide your writing and make the book a mess, write random letters over the ones you want to strike off. Not only will the new letters add a mysterious dimensions to the notes, but they can also make the letters more illegible and harder to figure out.

See for yourself http://i.imgur.com/AADKjFM.png

(AW- Anil)