Heroes star Hayden Panettiere dropped by Letterman to promote the third season of Heroes and her efforts to help save endangered whales.

Letterman's response to Hayden's compassion for the level of mercury in the whale and dolphins' system, "because of that factor, a lot of people will catch one and mount it on their walls as a thermometer." Ba dum bum.

The third season of Heroes premieres on September 22.