when you put a water bottle into a freezer, an let it sit there without touching it, sometimes it will still be water but when you disturb it it will freeze all at once. why does that happen?
It is because the liquid in the bottle is supercooled - the temperature of the liquid is below its normal freezing point, but the liquid has still not turned into a solid. That's because it needs something to kick-start the freezing process and encourage a small number of the liquid molecules to get together in a regular arrangement, as they do in a crystal, instead of moving around independently as they do in the liquid.

The process is called nucleation, because it encourages the molecules in the liquid to form a crystal-like nucleus onto which others can then latch. The kick-start can be given by a piece of dust, a rough spot on the surface of a container, or the shock wave generated when you hit a bottle just out of the freezer.
Could you design a machine that would supercool water on demand?
Sure thing, pal! 
Alright, so I'll just go over the basics again quickly. To supercool water, we need water (duh), a container and a system that would be able to cool this water below 0°C. Our design should be such, that it would not disturb that water in the container. Water expands when it freezes. Thus, our container should be as flexible as possible. To cool this container, we would need a box that works like a refrigeration unit. Hence, I would simply recommend getting a small refrigerator. To avoid disturbing the water in the container, I would recommend suspending the container in the box. I would also recommend suspending the box from a pole that would either be attached to the ground or to a vertical surface. 

So a quick recap of the design:
- A small refrigeration unit suspended from a pole (that is either attached to the ground or a wall) that can lower temperatures below 0°C.   
- A flexible container suspended in the refrigeration unit.