How does the anti-gravity fountain work? I just saw this video of an anti-gravity fountain on youtube, but I can't understand how you can reverse the flow of water. 
I know you can stop the flow of water by using something called a stroboscope, but how do you reverse it?
 A: It’s an optic illusion. A strobe light is used to illuminate the water droplets and the trick is very simple. If the light is strobing at the same frequency as the water droplets, the drops seem stationary because every time the strobe flashes it is illuminating the next water drop at the same location (in the drop) as the previous one. So changing the frequency to slightly slower will make the drops appear to fall in slow motion as the next drop gets illuminated (or arrived by the light) at a slightly lower position as the previous drop. And finally, if the strobe frequency is slightly higher it will ilumina-te the next droplet at a slightly higher position, thus giving the illusion that the drops are moving backwards. It’s just our eyes playing tricks.  
A: You already have a very nice answer, just let me point out that you probably have already seen this effect at work: sometimes very fast-moving wheels seem to move in reverse.
The flow in the fountain is not reversed, the droplets are falling. However, if you create a "periodic" system of droplets (in fact there are several droplets that are almost equally spaced), you can use the illusion given by the " wagon wheel effect ". After all, the periodic system of droplets is just an "unrolled wheel" and the effect is due to a sampling error of a periodic signal (every time the stroboscope light switches on your eyes take a "snapshot" of the droplet's actual configuration).
Here you can play with a periodic signal and see what happens when you modify the sampling rate.
Note#1: also the neon lamps have a discharge frequency of ~50 Hz, so they can create a stroboscope effect. Any machinery rotating at multiples of this frequency may appear to not be turning and this can give rise to safety problems in factories.
Note #2: if you really want to levitate water droplets, you may use something like an acoustic levitator.
