A practical joke is...well, anything but practical, especially to the victim, but the term apparently applies to a gag played on someone that involves something physical, rather than verbal. I've been a victim of a practical joke on occasion, and have been on the sending end as well.
Since my life more or less revolves around the trombone, it seems natural that "the true head of that family of wind instruments" (Berlioz) would be involved somehow in one of my favorites. Somewhere along the line I acquired an antique Conn 5H tenor trombone, a smallish silver-plated instrument with a beautiful sound. I was curious enough to write the C.G. Conn Corporation to inquire about it, and they responded in detail about the horn (this was before they inexplicably destroyed their historical records). It left the factory in 1914 (the exact date escapes me, but I have the letter somewhere, being of the opposite disposition of Conn regarding destroying or getting rid of anything). It was made before "A" was standardized at 440 Hz (or wherever the oboe wants to put itself that day), so it was outfitted with two tuning slides, "high pitch," defined by Conn as A 459, and "low pitch," A 440. The A 459 slide, pushed all the way in, effectively made it "Trombone in B Natural" (in contrast to B Flat, the standard).
There was certainly no practical use for a Trombone in B (it might. of itself, be considered a joke), but it occurred to me that it might be put to use in a practical joke. And lo and behold, an ideal situation presented itself. I was playing in my college marching band, a requirement for instrumental music education majors, and the band director approached me to say he wanted to feature the jazz band on a number in the upcoming half-time performance at Saturday's football game. This was before battery amps made electric basses possible on the field, and he wanted me to play the bass lines on Sousaphone for that one number.
We worked it out with one of the Sousy players who also played trombone, so that as we passed each other on the way to the formation featuring the jazz band, we would swap instruments; he played trombone on the one "big band" number, and I played the bass lines on Sousaphone. We then swapped back on the way to the next formation.
You can guess what happened next: we rehearsed all week and got the instrument swap really down, then on game day I took the Trombone in B on the field and transposed all my parts (I have a pretty good ear for that stuff). After handing off the trombone and taking the Sousaphone, I could hardly keep from cracking up, listening to my friend try to figure out what I had done to him, or probably more to the point, what was wrong with the horn. We subsequently switched back, and I continued to transpose so that my parts sounded normal. I never let on what I had done to him.
The trombone has been the butt of many a musical joke (What do you call ten trombones on the bottom of the ocean? A good start...), but I bet it hasn't been used that much on the other end of a joke. Trombone naysayers beware...