The greatest trick to play on the New Guy at work

I, too, have had some experience with the classic new guy gags (can of beep for the horn, blinker fluid for the turn signal, canvas stretcher, hole shrinker, light bulb repair kit, a hundred yards of flightline, etc.), but in my experience this one is both consistently hilarious and fairly foolproof. It's similar to the left-handed screwdriver gag listed above, but with a twist that safeguards against the New Guy seeing through the ruse. This probably won't come up unless your job is somewhat mechanical, but if it is, you'll have no trouble finding an opportunity to try it out. Also, if you're not in a country that uses the standard system of measurement, you might have a hard time pulling this off, but you could try switching standard for metric and vice versa.

The Metric Adjustable Wrench
While attempting to loosen or tighten some bolt or other, feign like the wrench you're using doesn't fit quite right. Mention that it's just a little off, but you don't want to strip the bolt. Tell the new guy you're going to need a metric adjustable and ask him if he wouldn't mind grabbing it for you.

At this point, usually the FNG will look confused for a second, which you should ignore by focusing your attention on the bolt or some other detail of whatever it is you're wrenching on. He'll head off for the wrench after a second or two. If he's a total rube, he'll go all the way to the toolbox/toolroom/store and look/ask for a metric adjustable, where he'll most likely be greeted by scorn, mocking, or at the very least the same confused look he gave you. If he's not a complete moron, he'll stop about halfway to his destination, think for a minute, then convince himself that you're goofing on him. Either way he comes back empty-handed.

This is where your acting skills come in. Now you have to act like he's the most incompetent person you've ever met before, how could he possibly not know what a metric adjustable is/looks like? Chew him out for wasting time and send him off again.

Now the FNG is pretty sure it's a goof, whether he's an idiot or not (though there are some truly gifted people out there that you may have to send out a few times). At this point, nine times out of ten he comes back with an adjustable wrench and tries to explain that since it's adjustable in an analog sense, it's neither specifically standard nor metric, and will work for any size bolt within its inner diameter.

This is the part that makes the metric adjustable gag so great. Once he's done expounding his theories on the universal application of all adjustable wrenches, tell him to read the handle of the wrench. Most American-made wrenches have their size marked on the handle, and the size is determined by the length of the wrench in inches. Common sizes are 8 inches, 12 inches, etc.

Once he's read the size off, cut him off quickly and say, "What was that?!?!"

He'll stutter for a second, then read it off again, "12 inches...."

To which you reply, "Exactly! INCHES, genius! Now go find a FRIGGIN' METRIC ONE! IT AIN'T ROCKET SCIENCE!" Wait for him to come back 10 to 20 minutes later, laugh uncontrollably while pointing.