There's really only one answer to that question in airport security.
Something about the x-ray didn't satisfy. He opened several compartments in my back pack until he found the zip-lock with cello accessories. Took a close look, then admitted, "We're just wondering what this is," pointing to my three-inch, five-fingered, rubber cello practice mute. Thankfully, my answer satisfied him and he didn't admit any further ignorance regarding cellos or their accessories. Not that I would expect a non-cellist to know what a mute is, but there are too many people who don't even know what a cello is! (Yes, just last month I was asked, once again, about my guitar!)
Then, I was telling my physiotherapist how little practicing I did while away - how my dorm neighbours didn't seem to appreciate good music, in spite of the use of the mute. "Cellos have mutes?" So, time to educate you masses ... at least the few of you who read my blog. And I know you're out there, in spite of the sparsity of comments. I do have a counter. At least the few of you who read my blog and are also cello accessory geeks or collectors of cello trivia?
So, here are my cello mutes.
The "Ultra" on the left is one of the best for quieting the cello. By pushing it onto the bridge, the vibrations are dampened so the sound just won't carry as far. I'll bet there are demonstrations on YouTube.
The smaller rubber "Tourte" and wire "paper-clip-style" (?? I don't know what it is really called) mutes are designed to hang out on the after-length when not in use, as you see in my picture, so that they can be quickly slid over the bridge as soon as the cellist sees "con sord." (con sordino) on the music, and removed when "senza sord." appears. It quiets the instrument somewhat, and changes the quality of sound, or timbre (TAM-ber).
You can see my wire mute is aging - discoloured, the tubing no longer as soft as it once was. It lives almost permanently in a box in my file cabinet now. I didn't put it on the strings for a picture because it really tugs on the strings and often requires re-tuning. Not sure about the Tourte, but the wire mute, tight on the strings, does also affect the sound slightly, even when not rolled up over the bridge. For me, it moves my wolf. Oh - that's an entire different blog post, which I'll probably never write due to my lack of understanding of physics.
I told my physio guy that there are four different kinds of cello mutes, but thinking longer, I'm thinking at least five. I think the ebony ones have been around forever, but I've not had one for my cello (did for my violin), and there are also metal practice mutes (wouldn't trust myself to not drop it on my cello!). A quick look at Cellos2Go.com shows me two more: a leather mute and a really pricey adjustable metal "My Mute" (because the other metal mutes are not a one-size-fits-all, I guess).
Sadly, while I found a video demonstrating violin mutes, there does not seem to be one with cello mutes. Violins get all the press ... and it seems an even bigger selection of mutes! Maybe because there is more reason to mute a violin! (Evil grin!)