This is a avery simple April Fools Joke. It can be played on anyone but the more intensely focused a person is the better. I used it on my manager and several others and I got everyone. Some play acting is required.
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
Take the cap off the pen and gently pull on the writing part of the pen it should come apart fairly easily.
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...
All you need from the second pen is the plug in the end of the pen. I gently pried my finger nail under the edge and it came right out.
Step 3: Assembly
Now take the plug from the second pen and press it into the empty end of the hollow tube from the first pen.
You can keep the actual pens for use as refills in other pens or deconstruct these after the joke and use them normally.