There are two ways to sense temperature.
The Thermister
The thermister comes in a wide variety of shapes and sizes.  This is one type of package that looks like a disc capacitor.

A thermister is a resistor that changes resistance in responce to temperature.
Simply get the thermister that increases resistance as heat increases.
They come in different sizes and shapes, as well as reactivity - how quickly it responds.  You will find a wide price range as well.
 In this setup, assume the fixed resistor is 47 K ohms.  Since this  thermister has low resistance when it is at 0o C, 1/11 of the voltage is used.  A Hi voltage still commands the inputs to gate 1.  The system is at rest.

At 10o C, thermister changes resistance to 100K ohms and 10/15 (2/3)  of the voltage is used up.  A lo voltage will trigger the inputs to gate 1, activating the system. 
 
The BiMetallic Strip
The bimetallic strip is frequently use in old thermostat regulators, and have a mercury switch on the end. 
picture of old thermostat regulator
 The mercury switch is a mechanical contact switch.