After both strips are heated, it is observed that Metal B expanded more than Metal A . This is due to Metal B having more electrons than Metal A to allow transfer of Thermal energy to take place more quickly in B. When transfer of thermal energy takes place more quickly, Metal strip B will expand first, pulling metal A along, resulting in a bend. A bend means one metal expanded faster than the other.
When cooled, it is observed that Metal B lose thermal energy at a faster rate than A since B had more electrons. This allowed Metal B to contract back to its original position first, pulling the bent Metal A along. Thereofore, the strip is straightened.