This is my first instructable, and the trick is pretty old, but anyways...

A good prank to pull on your co-workers, teachers, relatives, and for all ages, too!
It only takes about 30 seconds.

Materials:
Post-it or any tape that's opaque
The victim's mouse (computer laser mouse)
Pen
Scissors (optional)

WARNING: DO NOT DO THIS PRANK IF YOU GET PAPERCUTS TOO EASILY. Well, maybe not, but, be careful, although it doesn't require much effort.

Step 1: Writing on the Post-it/Tape

Write "April Fools!" or anything you want to write on the Post-it, like "PWNED!" or anything to tell the person that they've been pranked.

Step 2: Sticking Post-it on Mouse

Stick the Post-it on the bottom of the mouse, covering the laser.

If the Post-it is too big, trim it so it isn't visible when the mouse is flipped over.

Step 3: Flip It Over!

Flip the mouse over. The Post-it shouldn't be seen, if you can see it read Step 2 again, carefully.

Step 4: You Are Done!

Now, when the victim tries to use the mouse, he/she can't, because the Post-it is covering the laser.

Thank you for reading this, and I hope you enjoyed my first instructable!