This prank shall be dubbed "Word Swap", due to the fact that you'll be messing around with Microsoft Word's "Auto Correct" feature. This prank is slightly more risky, so please use at your own risk.

Requirements:

• Time (low) - less than 5 minutes, unless you are VERY slow.
• Cost (none) - none.
• Risk (medium) - even though this pranks is quite quick to pull off, you may cause your coworker to send something out that shouldn't be sent.

Steps:

1. Get to your coworker's computer and open up Microsoft Word.
2. Go to "Tools" then "Auto Correct". This should bring up the Auto Correct options. The location of these options may differ with different versions of Word.
3. Add a new entry for a word you want to mess around with. If you enter something like "the" it is nearly certain it will be used, however it may be a bit obvious for your coworker. Enter the word you want to replace that word with, for example, cuss words, "i suck", "i like poop", or anything else you think would be funny to see repeated over and over again. You may also want to try just replacing one letter in that word so that it looks like your coworker just misspelled a word over and over again.
4. Close word and wait.

Result:

As previously explained, this will cause whatever word(s) you chose to mess with to be replaced with the new words you entered. Depending on what you chose to replace, your results will vary. I would like to reiterate again that this is prank should be done with caution since you probably don't want your coworker to get fired because he or she sent a word document out to everybody with one of your new words in there. This prank should probably only be done on employees that you know won't be sending out any documents as such, or that must send them to your first perhaps.