Oh, yes, me too! I can scratch the inside of my wrist with the hand it’s attached to. And it’s not just my wrists that have loose joints - I can also put my hands on my hips and touch my elbows together (kills the shoulders!), bend my thumb over the knuckle where the index finger meets the back of the hand, and bend my elbows to a very painful-looking angle (maybe about 200 degrees).
I probably would be able to scratch the inside of my wrist as well if i didn’t have such short fingers!!

LOL
In order to impress / scare my friends and family, I have also developed a few other strange ways to make people cringe. I can move each eye independently of the other - train one eye on something unmoving and look in a different direction with the other eye. Scares my students half-to-death.
My finger is double jointed and I can hold it as if it’s stuck and bend just the fingernail part of it all the way up and down, and then pop my finger by moving it up and down, lol it freaks people out!! sorry too much info I guess…
I learned how to wiggle my ears from my grandfather. He described where the muscles were that I had to develop, and I practiced in front of the mirror for ages until I could make them wiggle.
I can flare my nostrils or wiggle them like a bunny. Fun for distracting people who should be paying attention in meetings.
I crack my kids and db with the nose flaring!! LOL
Lastly, I can hum and whistle at the same time. I developed this when I wanted to demonstrate the sound of two-part harmony to someone who couldn’t remember a tune long enough to repeat it to me while I harmonized with it. Unfortunately, I can’t actually harmonize with myself - I haven’t figured out how to adjust the pitch of one without affecting the pitch of the other the same way. When I have time (maybe when I’m a mom and have my hands full with my baby but my brain is aching for a meaningless project) I’d like to spend a few weeks practicing until I can really harmonize with myself.