Warning, this post contains immature language.

Don’t let April Fools Day catch you off guard this year—be prepared! I’ve got the perfect prank for you to play on your family, friends, and co-workers.

Chances are really good that everyone you know has at least one of the following: an iPhone, iPad, Mac, or PC. I’ve got a prank for you that you can use on all of those devices.

The AutoCorrect or text substitution feature built into OS X, iOS, and Microsoft Office can be an amazing productivity boost. But, it can also be used to freak out your unsuspecting family, friends and co-workers this April Fools Day.

On a Mac

In this example I’m using a Mac running OS X 10.7.3 (Lion), but it should also work on 10.6 (Snow Leopard). First, open the System Preferences app.

Launch the Language & Text Preference Pane.

Click on the plus sign (+) to create a new symbol or text substitution.

If I were your victim, you’d want to think of something I type all the time, My Name (where My Name is the persons name or whatever text you want to have AutoCorrect change), for example. Enter that in the Replace column.

Now, here’s the best part—in the With column enter the text you want to be replaced when your prankee types My Name. Because I’m so cool and sophisticated I would go with something like Poop.

Be sure to close the System Preferences app to cover your tracks. After you’ve added your entry to AutoCorrect it will be available in most of the apps on the Mac: Mail, iCal, TextEdit, Pages, Keynote, Numbers, etc.

Now, imagine the following scenario. I, your hapless target am completing a moving letter in TextEdit and have just typed My Name.

Bang, I’ve just been pranked!

Now, let’s see how to do this on a PC.

In Microsoft Office 2010

For this example I used a PC running Windows 7 and Microsoft Office 2010. To my knowledge it should also work on Windows XP or Vista using Office 2003 or greater.

Go to the File menu in an Office 2010 application, like Excel in this example. Click on Options.

Click on Proofing.

Then, click on the AutoCorrect Options button.

Similar to what you did on a Mac above, enter the text you want to change in the Replace column and what you want the substitution in the With column.

To keep things consistent we’ll go with My Name and Poop again. Click on OK to save your changes and then close out of the Options window.

Even though you added the AutoCorrect in Excel, it will be accessible on all Office 2010 programs including Word, PowerPoint, and Outlook.

So, when My Name is typed …

You get Poop.

On an iPad or iPhone

Lastly, let’s see how to add AutoCorrect entries to an iOS device—an iPad, iPhone, or iPod Touch.

Open the Settings app and tap on General

Tap on Keyboard.

Then, tap on Shortcuts.

To add an entry tap on the plus sign (+) in the upper right hand corner.

Entering AutoCorrect shortcuts is a little different in iOS. First, type in the Phrase or word you want to substitute. Again, we’ll go with our old standby, Poop.

Next, type in the Shortcut you want substituted. Please note that it must be one word only. So instead of My Name you’ll have to just use Name in this example.

Tap Save and close out of the Settings app.

The next time the recipient of your pranking prowess prepares a message in Mail.app and enters their Name they’ll get, you guessed it, Poop.

One quirk with iOS devices is that before the AutoCorrect occurs they may actually see a little popup before the text substitution happens. This would give them the option of tapping the little ‘x’ next to the substitution to cancel it. Given the proliferation of humorous AutoCorrect sites on the Internet, it’s a safe bet that your target will fall victim to your prank.