Some resistors exhibit a Negative Temperature Co-efficient (NTC) when heated and their resistance decreases.

Some resistors exhibit a Positive Temperature Co-efficient (PTC) when heated and their resistance increases.

A simple temperature detector is a diode.
The voltage across it decreases by 2mV per degree C. This is a very small change per degree C and must be amplified by a circuit to be useful.
A suitable circuit can be found in Learning Electronics Book 1 Page

1. This book is produced by Talking Electronics ($5.00). The circuit is a DIFFERENTIAL AMPLIFIER where two transistors are used to amplify the change in voltage from the diode. The circuit has been designed so that any change in temperature of one transistor will be negated by the other transistor and thus the junctions of the transistors do not affect the reading of the 0 -1mA meter connected to the output of the circuit.