In Microsoft Word, use the Autocorrect feature to "correct" a commonly used word such as "the" by automatically inserting "April Fools'!" It will drive them mad!

Chip in the Teapot
Submitted by Lorelei A.

I took the largest potato chip I could find and carefully placed it inside my mom's favorite china teapot.

If your mom is like mine, she will be very upset with you -- until you show her the potato chip. Be sure to say "April Fools'!" with a very big smile and a hug, as she has just been traumatized.