How to get sand to fall in minecraft?

Actualy there's a good way to do this in vanella minecraft
1. Make sure your platform is over void
2. Place a layer of tnt blocks
3. Place a layer of gravel (or sand) over the tnt
4 place stone preasure plates on the gravel (wooden ones will fall if items go on them)

if you don't want to use tnt it might be harder

Yeah there's a way if you put tnt under the sand and a pressure plate on top of the sand, when you walk on it it will set off the tnt (oh yeah it has to have nothing but void if possible under the tnt) and it will fall. If you want to make a game where someone has to knock the other players off then use the same technique but instead of pressure plates have the player who is the shooter have a bow with fire aspect so they can set off the tnt and knock the other players off.

Hope this helped and have fun.

Put water somewhere on the floor, make something like a pool, then put TNT on top of the water about 1 block away(water then nothing then TNT). Put sand on top of the TNT, then put pressure plates on top of the sand. There you have it.

Edit:
you walk on to the pressure plates on the sand and the sand will drop.

The TNT will not explode and break everything since it's in the water.