A bimetallic strip is comprised of two different metals. For it to be a bimetallic strip, the two metals must have different expansion coefficients. Brass and steel are common metals that are bonded together in a bimetallic strip.
When a bimetallic strip with brass and steel is heated, the brass will expand more the steel, causing the strip to curve with the brass on the outside. When the steel is cooled, it will curve with the steel on the outside. This works well in thermometers to help demonstrate the mercury rising as temperatures go up.Learn more about Thermodynamics