This can be hilarious if you get it right. A great prank for homework or when you are working on college assignments. The prank works in recent versions of microsoft word. You need to get on to your friend's computer when they are not looking.
Setting up the prank
In any open word document, deliberately miss-spell a word, put in a space after the word, and wait for the red squiglly line to appear. Then choose AutoCorrect and then AutoCorrect Options....
Then all you need to do is set up your funny automatic corrections.... Let your imagination run wild! Think of words which you will be using regularly in the document when your friend comes back. Names work especially well! Convert your own name automatically to "Mr Wonderful" or convert their name into an insult.
It is easy enough to reverse the prank by going back into autcorrect and deleting your new rules. But why bother - your friend may never work it out!