Paper 1 May/June 2018 PHYSICS (0625/12) IGCSE

The diagram shows four rods. Each rod is made of a different metal.
Wax is used to attach small metal balls at the rod ends P, Q, R and S.
Each rod is the same size. They are heated uniformly by a Bunsen burner at point X.
As the rods warm up, the wax melts and the balls fall off.
Why does the ball on the silver rod fall first?