Not all pranks have to be elaborate. Check out this prank that can be done in 30 seconds or less. Let's go! Grab a pen with a cap...
Step 1: Remove Cap
Do it. Remove the cap.
Step 2: Take Out Insides
This part may be a little tough. All pens are different, but on mine, I had to twist the tip of the pen to get the insides loose, then pull them out.
Step 3: Replace Cap
Put the cap back on as if nothing happened. Place fake pen in a place where it is likely to be used, and sit back and laugh.