Different materials expand differently when heated.
The thermal expansion coefficient is different for different materials. This demo shows this property.
aluminium foil, lamp
Cut two aluminium strips of almost 0.5 cm width. Roll the strip in the form of spiral. Shiny surface of one strip should face outside and that of the other should face inside.
Heat the spirals under a lamp. Spiral with inner shiny surface straighten up, whereas spiral with outer shiny surface shrinks.
Shiny surface of the foil is a thin metallic layer. When heated, it expands more and forces the strip to bend towards the paper.
This property of differential expansion is widely used in thermostats of household items like electric press, geyser etc.