2 BIC Pens (you know the 100 pack ones you can get at the drug store)
a victim or two or three or four
Step 1: Open the pen
At this point you have:
a pen filler
a pen cap
a hollow pen with one plug in an end of it
Step 2: The second pen...
Step 3: Assembly
You can keep the actual pens for use as refills in other pens or deconstruct these after the joke and use them normally.
Step 4: All finished... Now go find your victim
Finding your victim:
The best victim is an extremely focused individual.
ALSO try to get into theior office and hide all other pens this makes things really easy
Find several things you need to have them sign or mark up or something but state that it is very confusing and the they need to look at it carefully.
Hand them the documents and the 'pen' at the same time.
Generally the person will focus on the content and not even think about the pen.
When they get done 'examining the confusing document or sometimes even while they are examining it they will glance at the pen notice it is capped and uncap it moving the cap to the other end in preparation for signing.
They then go to sign and the see the pen is still capped so they uncap it again and move the cap to the other side and wallaaaa it is still capped.
Depending on how intensely focused on the confusing document they are, they may actually uncap the same hollow tube several times before they even catch on.
Note several of these pens can be placed in offices and the fun can last all day long.
You can even get the same person multiple times in this way.