  We calculate the tensor bispectrum mediated by an excited scalar field during
inflation and find that the bispectrum peaks in the squeezed configuration,
which is different from that of gravitational waves induced by enhanced
curvature perturbations re-entering the horizon in the radiation-dominated era.
Measuring the bispectrum provides a promising way to distinguish the stochastic
gravitational-wave background generated during inflation from that generated
after inflation.
