Two solid spheres, $P$ and $Q$, are initially at room temperature. Each sphere is heated so that its temperature rises by $15{}^{\circ }C$. The spheres are then allowed to cool back down to room temperature.

Sphere $P$ has a smaller thermal capacity than sphere $Q$.

Which sphere requires more thermal energy to be heated by $15{}^{\circ }C$, and which sphere releases more thermal energy as it cools back to room temperature?