The state an object is in depends on temperature[i]. At a very high temperature an object will be a gas and at very low temperatures it will be a solid.                                                                

The melting and boiling point is a number that we give to the temperate at which a solid melts or a liquid boils to produce a gas. If we look at water:

The melting point of water is 0⁰C and the boiling point of water is 100⁰C. This tells us that above 100⁰C water is a gas and below 0⁰C water is a solid. So therefore at temperatures greater than 0⁰C and less than 100⁰C water must be a liquid. 

[i] We’ll assume it is at a constant pressure or the pressure would change everything.