OMG LOOK AT THE TINY MUTE
Believe it or not, it is a functional mute, not a keychain or (really big) phone charm thing. You could clip it to your keys if you wanted to, but since it’s made of aluminum and therefor rather soft, it would get damaged very easily, so maybe don’t do that.
It’s called the Nano Mute, made by Best Brass, and is designed to fit harmon/bubble/extending tube mutes for trumpet and cornet (so far; perhaps one day there will be a trombone version too).
So if you have one of these:
...you can take out the tube-y bit and put the nano mute in the hole instead, and boom, now it’s a practice mute. One less mute to buy and/or carry around.
(Also supposed to be able to put a trp/cor/F.horn mouthpiece in the nano mute for quiet warm-up and lip exercises, but none of the mouthpieces we tested fit at all, so IDK what’s the deal there.)