The beauty of this prank is that it's quick, simple, easily reversed and quite effective. here's what to do:
Open Microsoft word on your target's computer and locate the auto-correct feature (my screen caps are on word for mac, but other versions will be very similar).
replace some key words like 'and' and 'the' with April Fool!' or something similar. click add after each word, making a list of words you've changed, so you get them all when you (or they) remove them. when you're done, click OK, and your all done! that was easy!
now just stand back and enjoy! What would JFK's Cuban Missile Crisis speech have looked like if he was pranked? check out the picture to see!