This is a really simple but very effective prank.

Step 1: Supplies

You will need:

Scissors

A small philips head screw driver

And, a cheap calculator with ruber buttons

Step 2: Opening the Calculator

Get your screwdriver and start opening the calculator.

Step 3: The Buttons

Take out the curcuit board so you can get to the buttons. Next get your scissors and cut out the clear and equals button, and switch their places. Make sure to get the buttons right side up.

Step 4: Finishing

Put the calculator back together with the two buttons switched. When some one does a problem on it, the answer will always be zero, provided they press 'equals'.

Step 5: Next Step

I only have a cheap small calculator, if you want to get more involved, you can get a more expensive calculator and mix around more key. this Instructable is only an idea you can add as many things as you like, you can change the three key to a B key by drawing one line, go crazy!
Thank you, yeturbumi for the idea