I'd like to take a moment to talk about The Three Seashells.
For those of you who haven't seen it, the movie Demolition Man (1993, starring Sylvester Stallone, Sandra Bullock, Wesley Snipes, etc.) is about a present-day police officer who is placed in suspended animation and awakened in a Utopian future. One of the gags in the film revolves around a bathroom implement, universally referred to as "The Three Seashells", that has come to replace toilet paper.
While no shots of the shells (either at rest or in use) are featured in the movie itself, a prop was constructed, and it looked like this:
While the way in which one uses the shells is not explained in the film, Stallone himself has gone on record as proposing that one uses two shells like chopsticks to remove large matter, the third is used in a scraping motion, and all three are then flushed.
This is wrong.
Whatever the function of the three seashells is, in less than forty years, they have come to universally replace both toliet paper and bidets. They must, therefore, be obviously superior to both toilet paper and bidets, and scraping one's butt with a chrome-finished disc clearly is not.
So, then... what do we know?
1) The three seashells have replaced toliet paper and water-based cleaning solutions to such an extent that backup paper is not provided. Their function is considered to be obvious and simple.
2) Human beings have been pooping and wiping their backsides - and innovating in regards to backside-wiping - for thousands of years. If this solution is so fantastic as to replace paper and water, it is logical to assume that it utilizes some form of technology that was only invented as recently as the twenty-first century.
3) While the technology in Demolition Man is presented as advanced, it's not quite Star-Trek-level, so things like targeted teleportation of fecal matter are probably off the table.
4) The three seashells are small, disc-shaped, and visually interchangeable, so they likely all do the same thing. They are always referred to as "The Three Seashells", not "The Seashells", so you likely need all three of them - not two, and not four - but, as humans only have two hands, you likely don't have to hold all three at once.
The shells in question are shown as scallop-shells, not conches or snail shells. So what are scallop-shells? They are small concave discs with one flat end - it is likely that they are used as small parabolic reflectors.
What can be parabolically reflected? Light, various forms of radiation, and sound.
Any light or radiation that would be strong enough to impact fecal matter would likely damage human tissue, so that leaves us with sound. This makes sense. After all, what is sound, if not vibration? And what is vibration, if not moving objects?
So now I get to talk about constructive interference.
Imagine two drummers. One hits his drum once every three seconds, the other hits his drum once every four seconds. So they're both hitting the drum - and producing a louder combined sound - once every twelve seconds, right? The same thing works with sound waves. If you have one speaker that's putting out ultrasonic vibrations at 15 kHz (i.e., 15000 "beats" per second), and another that's putting out ultrasound at 16 kHz, together you'd have a combined frequency of 1 kHz.
So imagine this: you've done your business. You reach over to the shelf beside the toilet, and you pick up two of the three seashells. As soon as they're picked up, they turn on. They use wireless power transfer (a technology we already have) to pull power from the home current. They buzz, slightly, in your hands.
You place each seashell on either side of you, pointing in towards your hips. The flat base allows you to stand them up, or perhaps you stick them to the walls or to your skin. You can feel the buzz slightly between your cheeks, like you're sitting on someone's electric toothbrush.
You pick up the third seashell and hold it in front of you, pointing inwards, towards your gut. Where the three focused beams of sound converge, you feel a resonance point - like a friendly but agitated bumble bee in your colon. The sensation is not unlike feeling your guts vibrate when you're near the speakers at a rock concert, but focused into a single intense node. You move your hand around, changing the angle of your third shell, and you feel the node shift around, sweeping around and through your body, a tingling sensation on and in your backside.
As the tingling passes over your dirty bits, you feel the fecal matter shake itself apart and throw itself off your skin, splattering noisily into the water below. After a few seconds of this, you judge yourself to be sufficiently clean - the tingling means it's working! - and put the shells back on the shelf, where they turn themselves off. You zip up, flush, and go about your business.
So, what are the benefits to the three seashells? Why are they superior to water and paper?
1) No waste of resources - only electricity is consumed.
2) Your hands never need to travel into the danger zone - there's no risk of bacterial contamination.
3) You don't even need to get up.
4) They can be easily used by the disabled.
5) Ultrasonic cleaning has been shown to use cavitation in fluids to kill bacteria.
So, is it realistic?
...to be honest, I have no idea. Given how fast and loose Demolition Man played with the science of cryogenic freezing, I'd say it's well within tolerance for the movie universe. If someone who knows more than I do about ultrasonic resonance nodes wants to mock up a working model, I'd certainly be first in line to try'em out.