A bimetallic strip is composed of two different materials that have different expansion coefficients. Let's say these materials are brass and steel, which have coefficients of linear expansion of

Brass: 19 x 10^-6 /°C

Steel: 11 x 10^-6 /°C

Say you have a strip of each material, and the strips have equal lengths at room temperature. The strips are bonded together into one bimetalic strip. When this is heated the brass expands more than the steel, and the strip curves with the brass on the outside of the curve. If the strip was cooled from room temperature instead it would curve the other way, with the steel on the outside.

Bimetallic strips are used as switches in thermostats.