Fig. 1 The pitch of liquid crystal.|
Fig. 2 A liquid crystal plastic film shows various colours after contacting a cup of hot water.|
What is "Liquid Crystal"?
We learnt about the three states of matter: solid, liquid and gas, in secondary school. However, in a broader sense, matters may rest in more than three states. In 1888, an Australian botanist Friedrich Reinitzer discovered that Cholesteryl benzoate has two distinct melting points. When this matter is heated, it melts from a solid state into an opaque liquid; if the temperature keeps increasing, it would turn into a clear and transparent liquid. Such an observation leads to the discovery of a new state of matter - liquid crystal - a substance half way between a solid and a liquid.
The structure of liquid crystals
As shown in Fig. 1, liquid crystals are formed from layers of rod-like molecules. Within a layer, the molecules have the same orientation; but the orientation differs across layers. Despite oriented differently, they maintain a spiral shape, i.e. adjacent layers form a particular angle. If we start at a particular orientation, after a certain distance, we would return to it. The distance required to return to the same orientation is called pitch (Fig. 1). When temperature increases, the angle formed by the different orientations increases, the spiral seems tightened and the pitch shortened, and vice versa.
Why do liquid crystal plastic films show various colours after contacting hot substances?
To understand the phenomenon, we have to note one thing first:
"Only the light waves with a pitch of an integral multiple of the wavelength could be reflected by liquid crystals." 
When we touch the plastic film, heat is passed to the liquid crystal, causing its pitch to shorten. When the pitch reaches a multiple of the wavelength of a certain colour light, we could see the light of that particular colour at the place while visible lights of other colours are absorbed (Fig. 2). Thus liquid crystals may indicate its surrounding temperature by means of colour. When different types of liquid crystals are combined into use, it could act as a sensor that detects various temperature differences, which could be used in checking cracks in an electric circuit board, the pattern of fluid motion, the state of a battery, the strength of radiation, or even the "mood-detecting rings" depicted in fictions.
 The detailed theory involves the concept of circular polarization which will not be introduced here.