Question

what allows water to change to all three states (liquid,gas,solid)

Answer 1

Hmmm! I'm far from being a science teacher, but I often change water (a liquid) into a gas and a solid in my kitchen.

Answer 2

Pressure and/or temperature depending on where you are on the phase diagram.

There is an example of the water phase diagram here:

http://en.wikipedia.org/wiki/Phase_diagram